Comparative Vertebrate Anatomy
Table of Contents
Chapter 1: Introduction page 3
Chapter 2: Concepts, Premises, and Pioneers page 8
Chapter 3: Protochordates page 14
Chapter 4: The Vertebrates page 20
Chapter 5: Development of the Vertebrate Body page 27
Chapter 6: The Integument page 41
Chapter 7: Skeletal Tissues page 59
Chapter 9: The Skull and Visceral Skeleton page 75
Chapter 10: The Appendicular Skeleton, Fins, and Tails page 95
Chapter 11: Myology page 108
Chapter 12: The Digestive System page 128
Chapter 13: The Respiratory System page 145
Chapter 14: The Circulatory System page 155
Chapter 15: The Urogenital System page 169
Chapter 16: The Nervous System page 185
Chapter 17: The Sense Organs page 208
Chapter 18: The Endocrine System page 224
Chapter One: An Introduction
1. Comparative Vertebrate Anatomy is the study of vertebrate structure (or
morphology) and the functional aspects of these structures.
a) It is a study of vertebrate morphology from an evolutionary perspective.
1) As a result, an understanding of phylogenies is essential (i.e.; an
understanding of the ancestral history of the vertebrate orders).
2) Evolutionary developments and relationships can be gleaned by looking at
embryological, and to a lesser extent fetal, development.
a] so ontogenesis is also very important.
B. The Phylum Chordata
1. The phylum chordata is made up of a group of animals all possessing four
a) These features are:
1) Notochord, a rigid cartilaginous rod defining the longitudinal axis in the
2) Dorsal Hollow Nerve Cord, the spinal cord an brain
3) Postanal Tail
4) Endostyle, a glandular groove in the floor of the pharynx
b) The phylum can be divided into three subphyla.
1) Urochordata, the tunicates and sea squirts
2) Cephalochordata, amphioxus
3) Vertebrata/Craniata, chordates having a vertebral column of bone or
2. The General Vertebrate Body Plan
a) Regional Differentiation
1) The typical vertebrate body has three regions: head, trunk, and tail.
a] The development of the head is a developmental process termed
1] The head contains the brain, a number, of special sense organs,
often jaws, and gills in fishes for respiration.
2] The trunk contains the Coelom, a body cavity that houses the
a} The coelom is surrounded by the body wall.
1} The body wall consists of muscle, vertebrae, and ribs
b} The trunks of the Gnathostomata, the jawed vertebrates and
most numerous vertebrates, will typically have paired pelvic and
pectoral appendages (ex; fins, wings, legs).
c} Many vertebrates have a Neck. The neck is a narrow structure
connecting the head to the trunk.
1} Necks are found in the tetrapods.
3] The tail is postanal in vertebrates meaning that it originates posterior
to the anus.
a} The tail is present all embryonic vertebrates but may be
lacking in the adult form.
b) Bilateral Symmetry
1) Bilateral symmetry means that the vertebrate body can be divided
into two equal right and left hand portions.
2) This allows vertebrate anatomy to be studied by dividing the body
a] There are three major planes of the body:
1] Transverse Plane (aka; horizontal plane, cross section) - a
plane running horizontally (left to right) dividing the body into
inferior and superior portions.
2] Sagittal Plane - a vertical plane dividing the body into left and
a} Midsagittal (or Median Sagittal) Plane - runs along the
midline of the body.
b} Parasagittal Plane - runs at other than the midline of the
3] Frontal Plane (aka; Coronal Plane) - a vertical plane which
divides the body into anterior and posterior portions.
c) Segmentation or Metamerism
1) Metamerism is the serial repetition of structures along the long
axis of the body.
a] It is evident in the embryo.
b] Metamerism is also present in the adult body to varying degrees.
1] Ex; muscle segments of fishes, rectus abdominis in humans, ribs
d) Notochord and Vertebral Column
1) The notochord is a cartilaginous rod that defines the long axis of
a] The notochord is the first skeletal feature to appear in the embryo.
b] It is located immediately ventral to the developing nerve cord
and superior to the digestive tube.
2) During development, in most vertebrates, bony or cartilaginous vertebrae
will grow around the notochord.
a] The weight bearing portion of the vertebra, the centrum, will
surround the notochord.
1] As to how much of the notochord will be retained in the adult
animal is variable.
a} Most fishes maintain the notochord in their adult form.
b} Some amphibians will show a pattern similar to that seen in
c} In reptiles, birds, and mammals the notochord is almost or
1} In mammals remnants of the notochord will remain as
portions of the intervertebral discs (called the Nucleus
2} In modern reptiles and birds even this vestige has been lost.
d} In the jawless vertebrate, the Agnathans, the notochord
grows and is maintained throughout life.
1} It will even develop extensions called Lateral Neural
Cartilages that will be found lateral to the spinal cord.
b] The Vertebral or Neural Arch will extend dorsally from the centra to
surround and protect the spinal cord.
c] A number of processes will radiate from the arch.
d] In some vertebrates a second arch, the Hemal Arch, will extend
from the centrum ventrally in caudal vertebrae.
1] The hemal arches surround and protect the caudal artery.
e) Dorsal Hollow Nerve Cord
1) The vertebrate nerve cord is dorsally oriented and hollow.
a] It consists of the brain and spinal cord.
b] The hollow center of the vertebrate nerve cord is termed the Neurocoel.
1] The neurocoel includes the central canal of the spinal cord.
2) The nerve cord originates by the process of Neuralation.
a] Neuralation typically occurs along the longitudinal axis of the embryo
dorsal to the notochord.
1] A groove, termed the Neural Groove, will form dorsal to the
a} The neural groove will sink into the embryonic body and
close off to form the Neural Tube.
b] In Agnathans and Neopterygians (the gars, bowfins, and teleosts)
neuralation has become slightly modified from the typical vertebrate
1] The neural groove does not form.
2] Instead a Neural Keel forms. The neural keel is a wedge
shaped ectodermal structure dorsal to the notochord.
a} It will separate from the surface ectoderm, form a cavity,
and become a typical nerve cord.
3) The nerve cord expands anteriorly to form the brain.
4) Cranial and spinal nerves will develop and radiate out from the
a] These nerves allow for communication between the CNS and the
rest of the body.
1] This is the job of the PNS.
1) The pharynx is a vital portion of the vertebrate embryo.
a] It shows the relationship between vertebrates and other chordates.
b] It produces a number of structures:
1] gills in fish
2] lungs in tetrapods
3] jaw skeleton and musculature
4] some endocrine glands
5] the middle ear in tetrapods
6] and serves a source of stem immune cells in the human fetus.
c] As a result, all vertebrate embryos will show a basic pharyngeal
2) The Development of Pharyngeal Pouches and Pharyngeal Slits
a] Pharyngeal pouches arise as outpocketings of gut endoderm.
1] These endodermal outpocketings will migrate towards the surface.
b] At the same time grooves in the abovelyng ectoderm, termed
Ectodermal Grooves, will grow towards the developing pouches.
c] Eventually the two structures will grow close together and will be
separated only by a thin membrane of tissue called the Branchial Plate.
1] If the branchial plate ruptures, a passageway forms termed
the Pharyngeal Slit.
a} In fishes pharyngeal slits are maintained throughout life for gills.
b} In tetrapods pharyngeal slits are only temporary
(a) Frogs have 6 pharyngeal pouches in the
(1) Four will form the gill slits of the tadpole.
[a] These four slits will close up again during
the metamorphosis into the adult frog.
(b) Chick embryos will have six pouches. Pharyngeal
pouches number 1, 2, and 3 will rupture and then close up
(c) In mammals only one or two pharyngeal pouches
will rupture. The rupturing pouches tend to be
anterior pouches and will close up again.
(1) Pharyngeal pouch number one becomes the
(2) Pharyngeal pouch number two will eventually
house the palatine tonsils.
(3) Several posterior pharyngeal pouches will give
rise to certain endocrine glands.
3) The Pharyngeal Arches
a] Between adjacent pharyngeal pouches/slits are columns of
tissue called the Pharyngeal Arches.
1] Each pharyngeal arch has four components or the blastemas
from which these components will arise. Typically these four
a} supportive skeletal elements
1} This skeletal component is termed the Pharyngeal or
b} skeletal muscles to operate the arch
1} This muscular component is termed the Branchniomeric
Musculature due to its relationship with the gills.
c} cranial nerve branches to innervate the arch
1} These cranial nerve branches will be both sensory and
motor in function.
2} The cranial nerves that send branches into the
pharyngeal arches are CN 5, 7, 9, & 10.
d} an aortic arch to connect the dorsal and ventral aorta.
2] These four components will also be found anterior to the first
pharyngeal arch and often posterior to the last pharyngeal arch.
3] The arches numbered from anterior to posterior.
a} The anteriormost arch is the First Pharyngeal Arch.
1} The first pharyngeal arch is typically referred to as the
Mandibular Arch since it contains the upper and lower
jaws and related structures.
b} The second arch is referred to as the Hypoid Arch.
c} The remaining arches are numbered 3 through 7.
1} Sometimes arches 3 through 7 are referred to as Branchial
Arches 1through 5 since they resemble unmodified gill arches.
g) Tube-Within-a-Tube Body Plan
1) Vertebrates, and many invertebrates for that matter, have a tube-
within-a-tube body plan.
a] The outer tube is the body wall.
b] The inner tube is the digestive tract.
c] The space between the outer and inner tubes, between the body
wall and digestive tact, is termed the Coelom.
1] The coelom in fishes, amphibians, and reptiles can be divided
a} the Pericardial Cavity which houses the heart
b} and the Pleuroperitoneal Cavity which houses the rest of
1} In tetrapods the lungs are located in this cavity.
(a) The pleuroperitoneal and pericardial cavities re separated
by a fibrous c.t. partition termed the Transverse Septum in
2] The coelom of birds and mammals is divided into:
a} The Thoracic Cavity having a Pericardial Cavity for the
heart and a pair of Pleural Cavities for the lungs.
1} In some there will also be a Mediastinal Cavity.
b} The Abdominopelvic Cavity having an
Abdominal/Peritoneal Cavity and a Pelvic Cavity.
3] The coelom is totally ensheathed in a series of serous
3. Other vertebrate Body Features
1) The integument is composed of the skin and the hypodermis.
a] The skin is composed of the Epidermis and Dermis.
1] The epidermis is a superficial layer composed of a many
layered epithelium having a number of functions.
a} It produces a variety of glands serving a variety of
b} It produces cornified appendages such as hair, feathers,
c} It provides protection from the external environment and
2] The dermis is deep to the epidermis and is a connective
a} In some animals it produces dermal bone.
b} It will also aid in the production of scales.
b) Respiratory Structures
1) Most vertebrates conduct external respiration by means of extremely well
a] These membranes will be either located in the pharyngeal
pouches (i.e.; gills) or will be derived from the pharyngeal floor (i.e.;
c) Digestive System
1) The digestive system consists of a digestive tube and a number of
a] The Digestive Tube/Alimentary Canal/Gastroinestinal Tract is a
long tube running from the mouth to the cloaca.
1] It has a number of regional specializations along its length
but the entire tube shares the same basic histology.
2] These specialized regions, or organs, include the oral cavity,
pharynx, esophagus, stomach, small intestine, and large
b] The Accessory Organs are located outside of the digestive tube
and release their products into the tube by means of ducts.
1] They include the liver, pancreas, and gall bladder.
d) Urogenital System
1) The urinary and reproductive systems are often considered as one
combined system, the urogenital system, due to their shared origins
a] Ex; the kidneys and gonads both develop close together on the
coelomic roof, and both the male reproductive and urinary systems
utilize the urethra in mammals.
2) The urinary system removes wastes from the body so as to maintain
3) The reproductive system allows for the propagation of the species.
e) Circulatory System
1) Vertebrates have a closed circulatory system whereby blood is
pumped by a muscular heart through a series of blood vessels.
1) The vertebrate skeleton is internal (an endoskeleton) and composed of
a] It forms a framework providing for the shape of the body, the
anchoring of muscles for movement, and protection of fragile
2) The skeleton can be divided into two portions:
a] Axial Skeleton is composed of the skull, vertebrae, and rib cage.
b] Appendicular Skeleton is composed of the bones of the limbs
and their associated girdles.
Chapter Two: Concepts, Premises, and Pioneers
A. Patterns and Processes
1.Homology is similarity due to common ancestry.
a) Initially all structures found in the same location with the same appurtenance
and function were assumed to be homologous.
1) Comparative Embryology has shown that not all look-alike
structures are homologous and some homologues bear no
resemblance to one another.
a] Ex; the human stapes bone and the hyomandibular cartilage of
sharks, the intermaxillary bones of human embryos and the
premaxilla of adult apes.
b] One way to test the veracity of suspected homologues is to use
1] If two structures (in two different animals) come from the
same embryonic precursor then they are homologs.
2] Another test is the structureʼs location and the evolutionary
relationship of the two animals in question.
b) The opposite of homology is homoplasy.
1)Homoplasy is similarity due to any other cause than common
c) Serial Homology is a term applied to segmental structures that as a unit
1) ex; spinal nerves of fishes and mammals.
2) However, one segment of the entire unit may not be homologous to
the similar portion of a metameric structure in another organism.
a] Serial homology is concerned with the entire metameric unit, not
the individual components.
2. Analogy is the term for two structures that have similar function but not
necessarily the same origin. It is a coincidental resemblance.
1) The wing of a fly and a bird are analogous while the wing of a bird
and a bat are homologous.
2) The horn of a bison and the antler of an elk are analogous.
b) Sometimes homologous structures have a similar function and are
1) Sometimes unrelated , nonhomologous structures have the same
function and are termed Analogous Homoplasies.
B. Adaptation and Speciation
1. Adaptation is the ability of a species to change due to some influence.
a) Biological Adaptation is a hereditary modification of a species structure,
or phenotype, to increase the probability of the species survival.
1) Since phenotype is typically the result of genotype, adaptations are
believed to be a change in the genetic make up of a species due to
a] Environmental factors result in genetic mutations that, if
beneficial, will be retained by the species.
b) Preadaptation is the presence of traits that enable a phenotype to
handle new environmental stresses prior to the to the development of
1) Example: Lungs were present in fishes prior to the movement of
vertebrates onto the land. The lungs served to allow some fish to
utilize atmospheric oxygen when dissolved oxygen in water was low.
2. Speciation is the development of two new species from one original species.
a) Often this involves the separation of one species into two reproductively
1) Over time, slight genetic differences will begin to accumulate and
the populations will begin to develop into two genetically distinct
a] This is termed Genetic Drift.
b) Speciation is typically the result of geographical isolation coupled with
genetic change but other factors can play a role.
1) Ex; Polar bears evolved from brown bears.
3. Convergent Evolution is a form of evolution where two or more unrelated
organisms acquire similar morphological features to exist in similar environments.
a) Ex; sharks, ichthyosaurs, and dolphins; bats, birds, and pterodactyls
b) The two species can exist concurrently or be separated by millions of
c) Convergent evolution produces similar features in two species that do
not share a common ancestor.
1. Development is useful to in the study of Comparative Vertebrate Anatomy
because patterns of development provide a further point of comparison among
a) Ex; Comparative embryology is used to test the veracity of
b) Heterochrony is a change in the timing of developmental events.
1) Ex; a delay in metamorphosis in some amphibians due to
2) Heterochrony can result in speciation.
a] One way is by causing the retention of juvenile characteristics
1] Ex; the axolotyls
2. Ontogeny and Phylogeny
a) Ontogeny/Ontogenesis is the developmental history of an organism. It is the
series of morphological events that occur from zygote to neonate, to adult, and
even to death.
b) Phylogeny is the evolutionary history of a group or Taxon.
1) Taxon is a related group of organisms 9ex; species, genus, family).
2) Phylogeny chronicles the changes that occur over the evolutionary
history of a group while ontogeny chronicles the changes that occur
during the lifetime of one individual.
c) There is an old saying:” Ontogeny recapitulates phylogeny.”
1) This means that as you watch the development of an individual, especially
embryologically, you are also watching the divergence of a species from its
2) However this is not quite true. Von Baerʼs Law better describes the
a] Karl Ernst Von Baer was an embryologist who noted that the features
common to all members of a taxonomic group develop earlier in ontogeny
than do features that distinguish subdivisions within the group.
1] So features diagnostic of the subphylum appear earlier in
embryogenesis than do features that differentiate families within the
b] Today Von Baerʼs Law has had a corollary added to it:
1] Features that develop earliest in ontogeny are the phylogenetically
oldest features, having been inherited from early common ancestors..
2] Also features that develop later are of more recent phylogenetic
a} An exception is the development of extraembryonic membranes
(ex; amnion) which develop later phylogenetically but arise early
ontogenetically due to the survival needs of the embryo.
D. Systematics and Taxonomy
1. Definitions of Classification
a) Systematics is the process by which organisms are grouped in
b) Taxonomy is the conventions used to apply names to these groups.
1) Taxonomy is still based on the Linnaean system established by
Carolus Linnaeus; although it has been refined.
a] It uses a hierarchical system and binomial nomenclature.
1] Hierarchical system: kingdom->phylum->class->order
2] Example of binomial classification; Canis lupus
2. Currently systematics group organisms based on their phylogentic
(or historical) relationships.
a) In the past classification involved groupings based on physical
similarities but that can be misleading.
1) Today Synamorphies are used.
a] Synamorphies are shared derived characteristics.
b) A phylogeny represents the current hypothesis as to the relationship
between species today.
1) These hypotheses are open to testing.
3. Cladistics is the methodology used by most systematics today.
a) Cladistics is based on the following assumptions:
1) The hierarchy of relationships is knowledgeable and can be represented in
by branching pattern called a Cladogram.
a] A cladogram is a useful tool to graphically display the relationships
between various organisms.
1] The cladogram consists of a branching pattern with terminal taxa at
the end of each branch.
a} Adjacent taxa represent sister taxa.
b} Where the taxa split is the location of a common ancestor (often
this common ancestor is hypothetical).
2] The branching pattern indicates relationships between taxa.
2) Only the characteristics that distinguish a group are useful in
a] ex; presence of vertebra is not useful to distinguish fish from
3) The distribution of derived characteristics on a branching diagram
determines homoplasy versus homology.
a] i.e.; The distribution of derived characteristics on a branching
diagram determines the independent development of similar
structures versus the development of similar structures due to
4) The distribution of characteristics follows the rules of parsimony: the
solution that is most economical is usually correct.
E. Organic Evolution and Evolutionary Selection
1. Organic evolution is based on the concept that organisms on earth have been
changing and that those species on earth today are descendants of earlier species.
a) This concept is partially based on fossil evidence.
2. The Evolutionary Theory
a) Jean Baptiste Lamarck (1744-1829) developed the concept of the
Doctrine of Acquired Characteristics.
1) It states that structures which are used over successive generations
become stronger and better developed while structures that are not
used over successive generations will degenerate and disappear.
2) Although it recognizes that species change over time it is not a
good mechanism for explaining change because it does not state how
those changes occur.
b) Charles Darwin (1809-1882) developed the Theory of Natural Selection
1) It was superior to Lamarckʼs idea because it provided a mechanism
to cause the changes.
Chapter Three: Protochordates
1. The term Protochordata is used to include to phyla of the chordates.
a) These two phyla are:
1) Urochordata – the sea squirts and other loosely related groups.
2) Cephalochordata – represented by amphioxus.
b) Both phyla are marine organisms that most likely share a common
ancestor with vertebrates.
2. Protochordates, vertebrates, and the most likely invertebrate relations the
echinoderms and hemichordates can be collectively grouped under the heading
a) Deuterostomata means “two openings” and refers to an event during
1) During embryogenesis a blastopore develops.
a]The blastopore develops into the anus and a second opening will form
during development that will become the mouth.
2) This pattern of development indicates the possibility of a common ancestor
for these disparate groups.
B. Basal Deuterostomes: Echinoderms and Hemichordates
a) The echinoderms first arose during the Cambrian Period and include 20 extant
classes such as sea stars, sea cucumbers, and sea urchins.
1) All echinoderms have a calcium carbonate skeleton, are marine, and most
species show radial symmetry.
a] However bilateral symmetry is a primitive characteristic of echinoderms.
b) The relationship between echinoderms and vertebrates has been
demonstrated by embryogenesis.
a) Hemichordates are a group of marine invertebrates commonly called “acorn
b) The relationship between the hemichordates and the vertebrates is widely
disputed. The hemichordates are a unique taxon.
1) Supporters of a close relationship cite the following items as evidence:
a] Hemichordates have a structure that resembles a dorsal hollow
1] It is a dorsally oriented strand of nerves that occasionally possesses
a} There is two problems with this, however:
1} The presence of the lumen is highly variable.
2} Hemichordates also have a ventral nerve cord (as do many
3} Hemichordates lack a true CNS.
b] Hemichordates have slits in their pharynx. These slits represent
gill slits and may be pharyngeal slits.
c] Hemichordates possess a structure called the Stomochord.
1] The stomochord may be homologous with the notochord.
a} The relationship between the stomochord and notochord
2] It is a short outpocketing of the foregut into the proboscis of
the acorn worm.
2) Developmental and molecular biological studies indicate a closer
relationship exists between hemichordates and echinoderms than
between hemichordates and vertebrates.
a] The larval forms are very similar.
b] Muscle proteins and other traits are close between the two.
1. Urochordates are a group of filter feeding, marine organisms.
a) Urochordates typically have a thin, transparent, nonliving outer tunic.
1) For this reason they are often also known as the tunicates”.
b) Since they possess a notochord in their tail, tunicates are considered to be
c) There are three classes of Urochordates: Ascidians, Larvaceans, and
1) Two of the three classes retain the larval form throughout life.
a] Only the ascidians mature into an adult form. They lose the tail and its
2. The Class Ascidians
a) Ascidians are tiny (between 0.5mm to 11 mm), short living (hours to
days) organisms found either in colonies or existing solitarily.
b) The larval form consists of a trunk and a muscular tail.
1) The tail is locomotory and possesses a notochord.
a] Running along the notochord are between 36 and 1,600 uninucleated,
striated muscle fibers.
2) The larval form does not feed.
a] As a result the viscera is reduced.
b] The larval form feeds off of stored calories.
3) The larval formʼs nervous system consists of a dorsal, hollow nerve
cord, several ganglia, and nerves.
a] It also has a primitive brain composed of a few ganglia with associated
sensory structures. These structures are:
1] Ocellus, a light sensitive receptor cell
2] Otolith, a receptor for stereoreception.
4) During respiration, water is drawn into the mouth and then passes
through the pharynx and over the gills lining the pharyngeal slits.
a] Then it passes into a chamber surrounding the pharynx called
b] Oxygen depleted water will exit the atrium via the Atriopore.
c) When the larval form metamorphosis into the adult form the following
1) Three adhesive papillae form to attach the sessile adult to a substrate.
2) The notochord is reabsorbed and used as a source of nutrition to supply
the energy required for metamorphosis.
3) The viscera changes.
a] The mouth becomes and incurrent siphon and the atriopore b
becomes and excurrent siphon.
1] This modification allows for filter feeding.
b] A ciliated, glandular groove develops on the floor of the pharynx called
1] The endostyle traps food and passes it on to the esophagus by
c] The ascidian also develops a short esophagus, stomach, and intestine.
1] Wastes are dumped into the atrium and released along with
deoxygenated water by means of the atriopore.
3. The Class Larvaceans
a) Larvaceans typically occur as free-floating plankton.
1) They are found in waters of less than 100m depth and below the level of
b) They are tiny being about 8mm in length.
c) They have a small body and a long tail supported by a notochord.
4. The Class Thaliceans
a) Thaliceans are free living or colonial organisms.
1) They alter between free living and colonial over succeeding generations.
b) Thaliceans resemble adult ascidians in appearance.
1) Like adult ascidians they lack a tail.
a] They propel themselves by squirting water out of the atriopore.
1. Cephalochordates are represented by Amphioxus.
a) They are also known as the lancelets due to the appearance of their
cylindrical bodies which taper at both ends.
b) Amphioxus are small, marine organisms found immediately off of sandy
1) They burrow into the sand leaving only their heads exposed so as to filter
2. Anatomical features of Amphioxus
a) The body consists mostly of a trunk.
1) The length of the trunk has repeating units of muscle termed Moyerʼs that
serve for locomotion.
a] The myomeres are composed of uninucleated, striated muscle fibers.
b] Adjacent myomeres are separated by a thin c.t. partition called a
b) It has a semitransparent skin composed of two layers.
1) There is an epidermis composed of a single layer of cells and a thin dermis
that facilitates cutaneous respiration.
2) The integument also includes a thin hypodermis.
c) Amphioxus possesses pharyngeal slits.
1) The pharyngeal slits do not open directly to the exterior.
a] Instead they open into the Atrium, a fluid-filled cavity that surrounds the
1] Food cleansed waters are pumped from the pharynx into the atrium
and expelled out of the body by a ventral opening called an Atriopore.
2) The slits do not serve a respiratory role but instead help to facilitate filter
a] Amphioxus uses the skin for respiration.
3) The number of pharyngeal slits can number up to 60.
4) They are reinforced by bars of cartilage termed Pharyngeal Bars.
d) Amphioxus has a notochord that runs from the rostrum to the tip of the
1) Unlike the typical notochord, the notochord of amphioxus consists of
sequentially arranged muscular discs.
a] These muscular discs have fibers that anchor the discs to a surrounding
notochord sheath composed of fibrous c.t.
b] Muscular contractions increase the stiffness of the notochord that may
help in digging.
e) The nervous system of amphioxus is similar to that of vertebrates.
1) It has a CNS consisting of a brain and spinal cord.
a] The CNS is covered by a vascular membrane called the
b] The brain of amphioxus has two subdivisions.
1] Vertebrate brains have three subdivisions.
2) Radiating from the spinal cord are spinal nerves.
3) Amphioxus does possess special senses but they are rudimentary.
a] Chemoreceptors are scattered over the surface of the body.
They are particularly concentrated in the anterior pharyngeal
region and mouth.
b] There also are light sensitive pigmented ocelli embedded along
the length of the spinal cord in the venterolateral walls.
f) Amphioxus feeds by filtering food out of the water.
1) Anterior to the mouth is the Vestibule, a structure designed o collect
a] The vestibule is surrounded laterally by the Oral Hood.
b] Caudally there is a perpendicular membrane called the Velum.
1] The velum is perforated by the mouth.
a} The mouth opens into the pharynx.
c] Cilia on the pharyngeal bars pull water through the vestibule,
through the mouth, and into the pharynx.
1] Within the vestibule is a mucus covered structure called the Wheel
a} The wheel organ has stubby projections to help trap food missed
by the mouth.
d] The opening of the vestibule is lined by Buccal Cirri which serve
to partially strain incoming water.
1] The buccal cirri have chemoreceptors to monitor incoming
2) Once food is in the pharynx it will be processed.
a] The processing of food requires specialized features of the
1] The pharynx has a ventral invagination lined with cilia and
mucus called the Endostyle or Hypobranchial Groove.
2] There is also a dorsal invagination lined by cilia and mucus
termed the Epibranchial Groove.
3] Connecting these two grooves are Pharyngeal Bands.
a} Pharyngeal bands cover the pharyngeal bars and are covered by
mucus and cilia.
b] Mucus secreted by these structures trap food.
1] The mucus and food become sticky mucus strands that are
propelled by ciliary beating into the epibranchial groove and
then pass caudally into the gut..
a} In the gut digestive enzymes will be added to the food strand.
b} Ciliary beating pushes the food-enzyme-mucus mixture into the
1} Some of the mixture is passed to the cecum for the
introduction of more enzymes.
2} The intestine terminates at the anus.
g) The coelom is reduced in adult amphioxus.
1) It becomes laterally compressed as the larvae elongates and more
pharyngeal bars are added.
2) As a result, only a trace coelom is found in the adult.
h) Amphioxus circulation differs significantly from that of vertebrates.
1) It lacks a heart and formed elements
a] The blood is a colorless serum.
2) One similarity to vertebrate circulation is that the amphioxus venous
pathways resemble those of embryonic vertebrates.
i) Cephalochordates lack an organized kidney.
1) Instead individual cellular components called Cyrtopodocytes filter
a] Cyrtopodocytes are intermediary between the Protonepheridia of
invertebrates and the podocytes of vertebrates.
j) In mature amphioxus, the gonads will be prominent and will bulge into
1) The gonads release sperm or eggs into the atrium.
a] The gametes are released from the atrium by the atriopore.
2) Amphioxus are Dioecious meaning that they develop only ovaries or
3. Amphioxus resembles the larval form of the lamprey called the
a) Initially the ammocoete larvae were actually classified as
1) One reason is that they stay in the larval state for a long time
(between 2 to 6 years).
b) Comparing and contrasting ammocoetes and amphioxus:
1) Ammocoetes have a notochord running from their midbrain into the tail.
2) Like amphioxus ammocoetes have a dorsal, hollow nerve cord and a brain.
a] Development of the nerve cord is the same for both.
b] One difference is that the ammocoete brain has three vesicles.
c] Ammocoetes have discrete special sense organs although they are
rudimentary by the typical vertebrate standard.
3) Ammocoete trunks also display myomeres.
4) Both organisms are filter feeders.
a] However in ammocoetes the endostyle has developed into a
b] Ammocoetes have a discrete liver diverticulum posterior to the pharynx.
5) Ammocoetes have a vascular arrangement that is more
reminiscent of vertebrates but, like amphioxus, it lacks a heart.
6) Ammocoetes have a kidney (although it is primitive).
c) The ammocoetes and amphioxus relationship argues for a common
Chapter 4: The Vertebrates
A. The Agnathans
1. The vertebrates can be divided up into two superclasses based on the
presence or absence of jaws.
a) The jawed vertebrates are called the Gnathostomata.
b) The vertebrates lacking jaws are called the Agnathostomata.
1) Agnathans are considered to be the most primitive vertebrate taxon
2) Today only two extant groups exist; lampreys and hagfish.
2. Although there are only two extant groups today, there are a large number
of extinct agnathans called the Ostracoderms.
a) Ostracoderms were a group of armored fish-like organisms of the late
Cambrian through the early Devonian periods.
1) Ostracoderms are the oldest known vertebrates with fossils dating
back to the Ordovician period and they are expected to have existed
prior to that during the late Cambrian.
b) Ostracoderm Anatomy
1) Ostracoderms were typically small marine species being between 2 to 3
cms in length.
a] However some species grew up to 1 m in length.
2) They are characterized by lacking jaws, lacking paired fins, and having a
body composed of overlapping plates of dermal bone.
a] The plates were largest on the head forming a bony shield and smaller,
more tile-like bones, on the trunk and tail.
3) Ostracoderms had three eyes.
a] There was a pair of upwardly oriented orbital eyes.
b] There was a third singular eye called the Pineal Eye.
4) They had a singular naris (nostril) which opened into a Nasopharyngeal
Duct connecting to an olfactory sac.
5) They had a small, jawless mouth.
6) They had rows of external gill slits.
7) Ostracoderms , along with the dermal bone, also had an endoskeleton
composed of bone and cartilage.
3. Extant Agnathans
a) The extant agnathans are hagfishes and lampreys.
1) The two groups are not really closely related and occupy two separate
a] Hagfishes belong to the class Mixini.
b] Lampreys belong to the class Cephalospidomorphi that also includes the
2) Extant agnathans share some common characteristics:
a] They have a prominent notochord that forms the axial skeleton
throughout the agnathanʼs life.
b] They lack a bony skeleton, scale, or dermal bony plates.
1] They lack bony jaws.
2] They lack a vertebral column.
3] These missing features may actually be derived characteristics that
evolved to maximize a life as a parasite.
c] Agnathans have fewer semicircular canals than do gnathans (which
1] Lampreys have two semicircular canals.
2] Hagfish have only one.
d] They had a singular naris that opened into a nasopharyngeal duct
connecting to an olfactory sac.
E] The mouth is surrounded by a Buccal Funnel.
b) The Hagfishes; Class Mixini
1) Hagfish are a class of marine, eel-like organisms.
2) They are bottom dwelling scavengers.
3) Hagfish have shallow buccal funnel surrounded by finger-like
papillae resembling short tentacles.
4) They have vestigial eyes covered by a thin, opaque membrane.
a] lampreys have better developed eyes.
5) The number of pairs of gill slits varies between species.
1) Lampreys resemble hagfish in appearance being eel-like aquatic
a] Most species are marine but some spend at least part of their lives in
b] Lampreys have changed very little from the time of the Carboniferous.
2) All lampreys are parasitic, feeding off of the blood of fishes.
a] They have a well developed buccal funnel with tooth-like structures that
anchor the lamprey on to its prey.
b] Lamprey have a muscular tongue coated with tooth-like denticles
designed to scrap away the flesh of the prey so as to feed on its blood.
3) Like ostracoderms, lamprey have a singular naris that opened into a
nasopharyngeal duct connecting to an olfactory sac.
a] Unlike ostracoderms, in lampreys the olfactory sac terminates blindly
into a Nasohypophyseal Sac.
4) All lamprey species have seven pairs of gill slits.
B. Superclasss Gnathostomata
1. Class Placoderm
a) Placoderms are the earliest of the jawed vertebrates existing
during the Paleozoic Era and primarily during the Devonian Period.
1) They are extinct.
2) They evolved from the ostracoderms.
b) Placoderms were named for their bony dermal plates.
c) They had paired fins; both pelvic and pectoral.
d) Placoderms species ranged in size from a few inches in length to a species
that reached 20 ft.
2. Class Chondrichthyes
a) Chondrichthyes is an ancient class and has two extant subclasses;
Elasmobranchii and Holocephali.
1) Elasmobranches have a cartilaginous skeleton.
a] The only bony component is found in the teeth and placoid scales.
2) Male elasmobranches have structures called Claspers which serve
as intromittent organs.
b) Elasmobranchs are the sharks and rays.
1) They first arose in the late Silurian Period and continue to exist
2) They have exposed gill slits, typically five pairs, and one pair of
c) Holocephalians are the chimera (aka; ratfishes).
1) They differ from elasmobranches in: an operculum that covers the
gill slits, and the teeth are replaced by bony plates.
3. Class Teleostomi; The Acanthodians and Osteichthyes
a) Group Acanthodia
1) The acanthodians are a group of fishes named for their hollow
a] Along with spines, bony plates also cover the body.
b] They had an operculum.
c]The skeleton had both cartilaginous and bony elements.
2) They are extinct having died out in the Carboniferous Period.
b) Group Osteichthyes
1) The osteichthyes are the ray-finned fishes.
a] They include both ancient and modern bony fishes.
1] The osteichthyes can be divided into the Actinopterygii and
2] They first arose sometime between the early Devonian and late
b] The group is named for their bony skeletons (although that is not
true for all).
1] They have membranous fins supported by bony rays.
2] They have an operculum.
3] They have scales.
2) Subgroup Actinopterygii; The Ray-Finned Fishes
a] Subclass Chondrostei
1] This is a primitive group of osteichthyes have few extant species.
a} Today represented only by the paddlefish and sturgeons.
2] They have an endoskeleton composed mostly of, but not exclusively
b] Subclass Neopterygii
1] The Gars and Bowfins
a} This division represents another primitive group of
fishes having few extant species, the gars and bowfins.
b} Unlike the chondrosteins, their skeleton is mostly ossified.
c} A distinct feature of this division is their scales that are
c] Division Teleostei
1] The teleosts are the modern fishes.
2]They are the most successful group of vertebrates today.
a} The teleosts make up 96% of all extant fish species and
have three suborders.
3) Subgroup Sarcopterygia; The Lobe-Finned Fishes
a] This is a group of bony fishes having a fleshy lobe to their paired
1] The sarcopterygians gave rise to the tetrapods.
2] They have internal nares that open internally into the
3] They retain a gas filled air sac that has the potential to
develop into a lung.
b] The sarcopterygians have two orders:
1] Actinistians, the coelacanths
2] Rhipidistians, the lungfishes.
a} This order includes the suborder Dipnoans which is the
group that gave rise to amphibians.
4. Class Amphibia
a) The amphibians are the first tetrapods.
1) They first appeared in the late Devonian and began to radiate in the
2) The class can be divided into two subclasses:
a] Labyrinthodontia were a group of early extinct amphibians.
b] Lissamphibia are the modern, extant amphibians.
b) Subclass Labyrinthodontia
1) The labyrinthodonts were the first amphibians.
a] The oldest known representative was Ichthyostega that appeared in the
2) Anatomical Features:
a] The labyrinthodonts are a diverse group but all share a similar pattern
on the apical surfaces of their teeth (which gave the subclass its name).
b] They share many features with rhidipstian fishes such as:
1] dermal scales
2] a fish-like tail supported by rays
3] rhidipstian-like skulls
4] and the presence of a lateral line, at least on the skull.
a} Today w see lateral lines in the tadpole but it is lost in the
adult form of extant groups.
3) One group of labyrinthodonts was the Anthracosaurs.
a] This group is believed to have given rise to amniotes in general
and to reptiles in particular.
1] They included the Seymouria that is believed to have lain
4) Another group of labyrinthodonts were the Temnospondyls.
a] Temnospondylian skulls display some features found in modern
amphibians (especially frogs) and is believed to be the ancestor of
d) Subclass Lissamphibia
1) These are the modern amphibians. There are three groups.
a] Order Anura/Salienta are the frogs and toads.
1] Anuran fossils date back to the Triassic.
b] Order Urodela/Caudata are the salamanders.
c] Order Apoda/Gymnophiona are the caecilians.
1] This is a group of tropical, limbless amphibians.
a} They can either be aquatic of burrowing.
2] The apoda may have a different origin than do the rest of the
5. Class Reptilia
a) The reptiles are the first Amniotes.
1) Amniotes include all of the mammals, birds, and reptiles.
a] Amniotes are characterized by:
1] the development of shell-covered eggs
a} This trait can be replaced over evolutionary time by becoming
b} This trait increased the ability of tetrapods to conquer land.
2] possession of extraembryonic membrane such as the amnion
a} These membranes allowed reptiles to lay eggs on land and avoid
an aquatic larval stage. They are:
1} The Amnion is a fluid filled sac that surrounds the embryo.
2} The Allantois
3) The Chorion
2) Along with the development of the amniotic egg, reptiles show other
features that increased terrestrial adaptiveness.
a] They are covered by scales composed of cornified epidermal
scales so as to decrease dehydration.
b] They developed a neck.
c] They have better developed limb girdles and claws on their
d] They developed the Metanepheric Kidney.
e] They display the partial or complete division of the heart into right
and left chambers.
3) Modern and ancestral reptiles are Ectothermic.
a] However they have given rise to endothermic tetrapods on a few
occasions such as mammals, birds, and possibly some dinosaurs.
c) Reptiles developed from a labyrinthodont, most likely an Anthracosaur,
during the early Carboniferous.
1) Based on skull structure, two reptilian lineages arose from a common
a] Synapsids are the lineage that gave rise to mammals.
1] The skull possesses one temporal fossa on each side.
2] They are believed to have evolved separately from the other reptiles.
b] Reptilia (or true reptiles or Sauropsida are all the modern reptiles, the
dinosaurs, and gave rise to the birds.
1] Reptilia can be divided into two subgroups based on skull structure:
a} Anapsids are reptiles whose skulls lack temporal fossa.
1}This is a primitive condition found in basal reptiles.
2} Today the only extant representatives are the turtles.
3} Anapsids gave rise to the diapsids.
b} Diapsids are all of the other reptiles.
1} Diapsid skulls have two temporal fossa on each side.
2} Diapsids are all of the extant reptiles (except for turtles) and
many extinct species. They include:
(a) Squamates; the snakes and lizards.
(b) Archosaurs; the dinosaurs, pterosaurs, and crocodilians
3} The diapsid reptiles gave rise to Euryapsid Reptiles that have
lost one pair of temporal fossa (only one temporal fossa on each
(a) Euryapsids were the plesiosaurs and ichthyosaurs.
6. Class Aves
a) Aves are bipedal, endothermic amniotes that are covered with
insulating feathers and have evolved for flight (although some have lost
1) The origin of birds is still controversial.
a] The prevailing hypothesis today is that they evolved from small
theropod dinosaurs or share a common ancestor with them.
1] Both groups share numerous anatomical features.
2] Some even classify birds as reptiles of the group Archosauria today.
2) Along with the abovementioned traits, birds have evolved other traits
a] lightweight bones many of which possess air sacs
b] loss of teeth
c] modified forelimbs
b) There are two generally recognized subclasses:
1) Archaeornithes are the extinct, ancient birds (ex; archaeopteryx).
2) Neornithes are the modern birds. There are three orders:
a] Odontognathae are a group of extinct, toothed marine birds.
1] There only two known representatives ichthyrinis and hesperornis.
b] Palaeognathae (aka; ratites) are a group of flightless birds (ex; ostriches
c] Neognathae (aka; carinates) are all other modern birds.
1] Neognathans have a large keel, or carina, for anchoring flight
7. Class Mammalia
a) Mammals are endothermic amniotes covered by insulating hair and
producing milk for their young.
1) Mammals evolved from synapsid reptiles.
a] Synapsid reptiles diverged from all other reptiles very early in
reptilian history (in the early Carboniferous).
1] The earliest synapsid reptiles were the Pelycosaurs.
2] The pelycosaurs evolved into the Theraspids that gave rise to
a} There are a number of features found in modern mammals that
were present in therapsids:
1} such as 2 occipital condyles, a secondary palate, and
b] Mammals first appeared in the Triassic period.
1] Modern mammals still display a synapsid skull.
2) Other features of mammals include:
a] 3 auditory ossicles
b] a muscular diaphragm separating the thoracic and
c] absence of a cloaca in the adult in all mammals excepting the
d] the vast majority possess sweat glands
e] presence of heterodontic teeth with adult and juvenile dentitions
1] The homodontic teeth of cetaceans are a derived
f] loss of the fourth aortic arch
g] a single dentary bone on each side of the lower jaw that will
articulate with the squamosal bone
h] pinna to gather sound waves
i] a more specialized larynx
j] extensive development of the cerebral cortex.
b) Based on reproductive strategy there are two groups of mammals.
1) Prototheria (or Monotremes) are egg laying mammals.
2) Metatheria are mammals that give rise to live young. There are two
a] Metatheria (or Marsupials) give birth to almost larval young that
migrate to, and finish development in a pouch.
1] They lack a placenta.
A} Instead a yolk sac serves as the role of the placenta.
b] Eutheria(or Placental Mammals) have young that receive
nourishment via a placenta while in the womb.
Chapter Five: Development of the Vertebrate Body
A. Vertebrate Eggs
1. Vertebrate eggs vary in the amount and location of their yolk.
a) The amount of yolk present causes vertebrate eggs to be divided into 3
1) Microlecithal Eggs are eggs having very little yolk.
a] Microlecithal eggs are found in amphioxus and therian
2) Mesolecithal Eggs are eggs having a moderate amount of yolk.
a] Mesolecithal eggs are found in freshwater
lampreys, actinopterygians, gars & bowfins, lungfishes, and
3) Macrolecithal Eggs are eggs having a lot of yolk.
a] Macrolecithal eggs are found in marine lampreys, teleosts,
reptiles, birds, and monotremes.
b) Yolk distribution occurs in two patterns in vertebrate eggs:
1) Isolecithal is the pattern where the yolk is evenly distributed
throughout the egg.
a] The isolecithal pattern tends to be found in eggs having a
microlecithal yolk content.
b] Isolecithal eggs are found in therian mammals.
2) Telolecithal is the pattern where the yolk is unevenly distributed.
a] This results in the egg being divided into two poles.
1] The Vegetal Pole is a large yolk mass.
2] The Animal Pole contains the embryo.
b] The telolecithal pattern is found in eggs having a macrolecithal
or a mesolecithal yolk content.
c] Telolecithal eggs are found in all nontherian vertebrates
( including amphioxus).
2. Oviparity and Viviparity
a) Oviparous animals lay eggs.
1) The eggs contain all of the nourishment necessary for the development of
a] If there is not enough yolk, as is the case with amphibians, the young
will hatch in a larval form.
b) Viviparous animals give birth to live young.
1) There are two patterns of viviparity based on the source of
a] Ovoviviparous - nourishment comes form yolk as in oviviparous
1] The egg hatches shortly before birth or very shortly after birth.
a} The motherʼs body provides protection for the egg but does not
provide nourishment or oxygen.
2] Found, for example in Squalus sulkeyi.
b] Euviviparous – nourishment is supplied by the mother.
1] Maternal tissues supply nourishment and oxygen and also
2] ex; placental mammals
c] There are a variety of intermediary forms.
1] For example, some reptiles nourish the embryos with yolk
early in the pregnancy and then by maternal tissues later in the
3. Methods of Fertilization
a) Fertilization can be either external or internal.
b) Internal fertilization occurs in species that are viviparous or are
oviparous species that produce hard shelled eggs.
1) The male will have an organ designed to introduce sperm into the
the female reproductive tract called an Intromittent Organ.
c) Oviparous animals that produce soft shelled eggs will use external
1) External fertilization occurs in most fishes and anuran amphibians.
2) One exception are urodeles and apodans which utilize internal
fertilization even in oviparous species.
3) Due to the needs of external fertilization, reproduction will take place
in an aquatic environment.
a] To compensate for the diluting effects of water, a copious
amount of sperm is released over the eggs as they are being laid.
B. Early Development in Representative Chordates
1. From Zygote Stage Through the Blastula Stage
a) The Zygote is the fertilized egg, the first embryonic stage.
1) Once fertilization has occurred the zygote immediately begins to
undergo a special form of cell division termed Cleavage or
a] Unlike typical cell division, cleavage lacks a resting period
between mitotic events meaning that there is no time to allow for
the recovery of cytoplasm.
1] As a result, each time the cell divides, the two daughter cells
are smaller than was the mother cell.
a} So the eight celled embryonic stage is roughly the same size as
is the one celled zygote.
b) The embryo will become subdivided into smaller and smaller cells until
forming a hollow ball of cells termed the Blastula.
1) Each cell of a blastula is termed a Blastomere and the hollow center
of the blastula is called the Blastoceol.
2) Patterns of Blastula Development
a] In microlecithal eggs the blastomeres are of approximately equal
b] In mesolecithal eggs the increased yolk content (compared to
microlecithal eggs) interferes with cleavage resulting in
blastomeres of unequal size.
1] Cells at the animal pole will divide faster than will those at the
a} As a result, the cells of the animal pole will divide faster and be
smaller than will those of the vegetal pole.
c] In macrolecithal eggs there is even a higher yolk content causing
even more interference with cleavage.
1] As a result, cleavage is restricted to the animal pole.
a} This results in a group of cells called a Blastoderm resting on a
large aggregate of yolk.
d] Therian mammals have microlecithal eggs with very little yolk
and do not show the typical vertebrate pattern of animal and
1] As a result therian mammals show a unique cleavage pattern.
a} Cleavage starts out as in other vertebrates but will show
divergences as it progresses.
1} So the blastula in therians is given a special term, the
b} Cleavage will result in a group of cells located within the
blastocoel appearing during the 16 cell stage called the Inner Cell
c} By the 32 cell stage the outer layer of the embryo has expanded
to form the Trophoblast.
1} The trophoblast can absorb uterine secretions to nourish the
inner cell mass prior to the establishment of the placenta.
2} The trophoblast will develop into the placenta and the
a) Gastrulation is a series of cellular movements in the embryo resulting in the
formation of the three primordial germ layers.
1) The three primordial germ layers are: Ectoderm, Mesoderm, and
2) During gastrulation the embryo will go from a hollow ball of cells to a three
layered embryonic stage in which bilateral symmetry is established.
3) These cellular migrations are called Formative ,or Morphogenetic,
Movements and are controlled by chemical agents termed Morphogens.
b) Gastrulation in Amphioxus, An Animal with a Microlecithal Egg
1) Beginning with the blastula: a group of blastomeres, termed Presumptive
Endoderm, invaginates into the blastocoel.
a] This process is called Involution.
b] Involution causes the formation of the Archenteron, the primitive gut.
1] This embryonic stage is called the Gastrula.
2] The entrance into the archenteron is termed the Blastopore.
a} Cells from the blastopore migrate into the gastrula to begin the
formation of the notochord.
1} As a result, these presumptive endodermal cells are
termed Presumptive Notochord Cells.
2} The early forming notochord will temporarily form the roof of
c] Presumptive Mesoderm Cells will begin to aggregate lateral to
the newly forming notochord and become undifferentiated
d] At this stage the amphioxus embryo is composed of an outer
layer of ectoderm and an inner layer of endoderm surrounding the
1] The roof of the archenteron has the notochord flanked by
2) Once the mesoderm has become established, the undifferentiated
mesodermal cells will be arranged in bands.
a] These mesodermal bands will fold and move upwards due to the
driving force of mitosis that is still occurring rapidly.
b] The upwardly folding mesoderm will form Mesodermal Pouches
(aka; Coelomic Pouches) progressing from the anterior to the
posterior of the animal.
1] Each mesodermal band will produce its own mesodermal
2] At this point, the amphioxus larva has hatched and is
swimming about in its environment.
c] Once the mesodermal pouches form, they begin to extend
ventrally between the ectoderm and endoderm.
1] The right and left mesodermal pouches will eventually extend
to and meet at the ventral aspect of the larva to form the
d] The fate of the mesoderm in the mesodermal pouches:
1] The outer wall of each pouch lays against the ectoderm and
is now termed Somatic Mesoderm.
a} Together with the ectoderm somatic mesoderm makes up
the body wall or Somatopleure.
2] The inner wall lays against the endoderm and is now termed
3] The coelom is the space between the splanchnic and somatic
3) The development of the mesodermal pouches occurred in a segmental
pattern in amphioxus.
a] The pouches on the right and left hand sides will fuse ventrally.
b] Later the fused pouches will begin to fuse to one another between
adjacent segments starting anteriorly and progressing posteriorly.
4) The germ layers are now established and Organogenesis (the formation of
organs and organ systems) can occur.
c) Gastrulation in Frogs, An Animal with a Mesolecithal Egg
1) In vertebrates other than therian mammals yolk in the vegetal pole affects
a] In frogs a process called Epiboly occurs.
1] The small, rapidly dividing cells of the animal pole will grow
downwards over the large, slower dividing, cells of the vegetal pole.
2] This group of rapidly dividing cells will flow inwards, over the yolk
forming a blastopore and an archenteron. This is called Involution.
a} The yolk cells will make up the floor of the archenteron.
b} The rest of embryogenesis is similar to that seen in amphioxus .
2) Notochord and Dorsal Mesoderm Formation in Frogs/Mesolecithal
a] As involution progresses, a stream of undifferentiated cells will
flow over the dorsal lip of the blastopore.
1] These cells establish a narrow band on the roof of the archenteron
called the Chordomesoderm.
a} A notochord develops from the midline of the chordomesoderm
and will temporarily remain on the roof of the archenteron.
b] The chordomesoderm cells lateral to the developing notochord
will give rise to a pair of undifferentiated mesodermal bands (lateral
to the notochord).
1]These bands of mesoderm form the dorsal mesoderm.
2] The dorsal mesoderm will become segmental and form
hollow mesodermal somites.
a} The somites will form the musculature of the body wall.
3) Endoderm and Ectoderm Development Formation in
a] While the notochord and dorsal mesoderm are developing,
presumptive endodermal cells will migrate into the interior of the
embryo, spread out, and provide (with the yolk cells) the lining of
the archenteron. These cells will be the endoderm.
b] The cells on the surface of the gastrula become the ectoderm.
4) Intermediate and Lateral Plate Mesoderm Development Formation
in Frogs/Mesolecithal Eggs
a] Mesodermal cells (other than those in the somites) will migrate
into the gastrula through the blastopore.
b] These mesoderm cells expand out as a sheet moving cephalad
between the ectoderm and the endoderm.
1] This mesoderm is termed Lateral Plate .
2] Lateral Plate Mesoderm will divide into two sheets:
a} Somatic Mesoderm will be the outer sheet.
b} Splanchnic Mesoderm will be the inner sheet.
c} The coelom is the space between the somatic and splanchnic
3] Somatic mesoderm and the ectoderm together form the
4] Splanchnic mesoderm and the endoderm together form the
5] A portion o the lateral plate mesoderm does not follow this
a} This portion of the lateral plate mesoderm is located dorsal to the
rest of the lateral plate mesoderm and immediately lateral to the
1} These cells will give rise to much of the urogenital system.
2} This mesoderm is called Intermediate Mesoderm or
d) Gastrulation in Birds, An Animal With a Macrolecithal Egg
1) Due to the greater yolk content of macrolecithal eggs, epiboly can
a] Remember cleavage does not occur in the yolk in macrolecithal eggs.
1] Only the cells of the animal pole will demonstrate cleavage and give
rise to a Blastoderm.
b] The blastoderm organizes into an upper sheet of cells (the Epiblast) and
a lower sheet of cells (the Hypoblast) separated by the blastocoel
forming a Bilaminar Embryonic Disc.
1] Cells from the periphery of the hypoblast will grow downward over
the yolk sac.
a} They will become part of the endoderm lining the yolk sac.
b} The hypoblast cells do not become a portion of the developing
1} Instead the hypoblast cells are limited to the yolk sac and its
2] The cells of the epiblast will give rise to all of the structures
of the developing embryo.
2) Notochord and Mesoderm Formation in Birds/Macrolecithal Egg
a] Cells from the epiblast develop into a Primitive Streak.
1] The primitive streak is a thin, multilayered structure running the
length of the embryo.
2] A thickened nodule of closely packed blastoderm cells forms along
the primitive streak called Hensenʼ's Node.
a} Hensenʼs node defines the caudal end of the future embryo.
3] The primitive streak and Hensenʼs node are the functional
equivalent of the blastopore in eggs having less yolk.
b] Epiblast cells migrate from the primitive streak into the hypoblast
where they will displace the hypoblast cells to establish the
1] Epiblasts cells will then migrate a second time. These cells
will situate between the epiblast and endoderm to form the
2] The Remaining portion of the epiblast is now the ectoderm.
3] The result of these migrations is the formation of a Trilaminar
c] A Notochord Process develops from the primitive steak and
pushes through the epiblast.
1] Other cells move forward along the newly forming notochord
to become the dorsal mesoderm.
a} Simultaneously the lateral plate mesoderm streams in
between the ectoderm and endoderm.
2] The dorsal mesoderm will eventually segment to form
a} Simultaneously the lateral plate mesoderm differentiates into
somatic and splanchnic mesoderm.
d] The splanchnopleure will extend down over the yolk to form a
1] By the second day of incubation the splanchnopleure
produces the first blood cells and a delicate network of Vitelline
Vessels (aka; Omphalomesenteric Vessels) on the Area Opaca
of the yolk sac.
a} Vitelline vessels carry yolk globules into the embryo.
1} Carries yolk to the early, “S” shaped heart of the embryo for
e) Gastrulation in Placental Mammals
1) Gastrulation begins with the Blastocyte, a hollow ball of cells having
a blastocoel and being divided into a trophoblast and an inner cell
a] The trophoblast does not contribute to the embryo, It contributes
to the extraembryonic membranes.
1] It will establish contact with and implant in the uterine wall.
b] The cells of the inner cell mass will develop into embryonic
2) Using a variety of mechanisms in different placental mammals, the
inner cell mass will divide into a Bilaminar Embryonic Disc/Blastodisk
having an epiblast and primitive endoderm.
3) Then epiblast cells will migrate into the space between the epiblast
and primitive endoderm to form the mesoderm and a Trilaminar
a] The epiblast cells at this point will become ectoderm.
b] Mesoderm cells will develop into dorsal mesoderm, forming
along the developing notochord, and lateral plate mesoderm.
1] The lateral plate mesoderm will develop into somatic and
4) Notochord formation is similar in mammals as it is in macrolecithal
eggs which demonstrates our reptilian ancestry.
5) In placental mammals the three gem layers begin to form at the
same time as the extraembryonic membranes begin to form.
a) Neural tissue develops from the ectoderm.
1) Presumptive neural ectoderm I found in a wide band on the dorsal
aspect of the embryo laying above the notochord.
a] As the gastrula elongates, this band thickens forming the Neural
b] The neural plate develops a pair of folds called Neural Folds on
either side of the Neural Groove.
1] The formation of the neural folds vs. the neural groove is due
to different mitotic rates.
2] At this point the embryo is now termed a Neurala.
3] The neural groove is widest at the anterior extremity of the
a} This wider region will become the brain.
1} The remainder of the neural groove will form the spinal cord.
c] The neural plate will sink deeper into the neural groove and the
two neural folds will grow closer together.
1] Eventually the neural folds will meet over the neural tube and
a} Now the neural groove is a Neural Tube.
1} The cavity enclosed by the neural tube is called the
(a) The neurocoel will give rise to the ventricle system of the
brain and the central canal of the spinal cord.
2) As a result of the developmental processes involved, the CNS is
ectodermal, dorsal, and hollow.
b) Neuralation will vary a bit between different vertebrates. The
differences are minor overall.
1) One of the greatest differences is seen in agnathans and
actinopterygians where neural folds do not form.
a] Instead the neural ectoderm becomes a wedge-shaped structure
called a Neural Keel.
1] The neural keel will push down into the body above the
2] The neural keel will then detach from the surface and cellular
rearrangements will allow for a neurocoel to form. This gives it
a structure similar to that in the typical vertebrate CNS.
a) Mesenchyme is an undifferentiated embryonic tissue.
1) It is composed of totipotent cells called Mesenchymal Cells residing
in an amorphous extracellular matrix.
a] Mesenchymal cells can develop into any other cell type.
1] They have branching, dendritic processes.
2) Mesenchyme makes up much of the early embryonic body.
3) Most mesenchyme is of mesodermal origin.
a] Some, called Mesectoderm, is of ectodermal origin.
b) During organogenesis mesenchyme will become organized into clusters
1) These blastemas will gradually assume the basic outline of future
organs and their mesenchymal cells will differentiate to from the
c) Some Mesenchymal cells are present in the adult.
1) They will be pleuripotent in nature allowing them to replace
damaged/lost cells so as to repair tissues.
C. The Fate of the Ectoderm
1. Ectoderm will give rise to:
a) CNS,PNS, and some of the associated membranes,
b) sensory epithelium of the special senses and the retina of the eye,
c) epidermis, epidermal derivatives, integumentary glands,
d) the lining of the stomodeum and the proctodeum and their derivatives,
e) numerous other tissues especially those of the head and pharynx.
2. Stomodeum and Proctodeum
a) The oral cavity begins to form as a midventral invagination of the
ectoderm at the anterior end of the foregut on the embryonic head.
1) This invagination is called the Stomodeum.
2) The foregut and stomodeum are initially separated by a thin partition
called the Oral Plate.
a] The oral plate will rupture to allow for communication between
the digestive tract and the external environment.
3) The stomodeum ectoderm lines the anterior portion of the oral cavity
including at least part of the tongue.
a] Some of this stomodeal ectoderm will be invaded by endoderm
making it hard to distinguish which cells produced which of the
glands located at he junction of ectoderm and endoderm.
b] In mammals stomodeal ectoderm gives rise to cells that secrete
1] In other vertebrates neural crest cells secrete enamel.
4) An evagination called Rathkeʼs Pouch forms from the roof of the
stomodeum prior to the rupturing of the oral plate.
a] Rathkeʼs pouch will give rise to the adenohypophysis.
b) The proctodeum is another ectodermal invagination (like the
stomodeum) but it is associated with the hindgut.
1) The proctodeum and hindgut are separated by a thin partition called
the Cloacal Plate temporarily.
2. The Neural Crest
a) When the neural tube formed some of the neighboring ectoderm
separated from the surface ectoderm and the neural tube becoming the
1) The neural crest first appears as neuroectodermal clusters.
a] One pair of neural crest will occur in each metamere and several
clusters will occur lateral to the hindbrain and midbrain.
b) Neural crest cells proliferate rapidly and give rise to a great number of
1) Some cells in the head form blastemas for the skeleton of the
arches including the jaws.
2) Some form portions of the neurocranium and some of the
membrane bones that will surround the neurocranium.
3) Some are involved in the development of the endocrine glands (The
thyroid, parathyroids, and thymus) and glands derived from the
pharyngeal/branchial pouches called Ultimobranchial Bodies.
4) In agnathans, neural crest cells will give rise to the branchial basket
and lingual cartilages.
5) neural crest mesenchyme in the head or the trunk moves ventrally
around the notochord to form the autonomic ganglia and adrenal
6) Neural crest cells around the neural tube produce the meninges
(excepting the dura mater).
7) Some neural crest cells will become a variety of pigment cells
including those of the skin.
8) Some neural crest cells remain close to their point of origin to
become the perikarya of sensory neurons whose perikarya are located
in PNS ganglia.
9) Other neural crest cells will migrates distally along the course of the
PNS to give rise to the neurilemma.
3. Ectodermal Placodes
a) Ectodermal placodes are paired thickenings of localized portions of the
ectoderm that sink beneath the skin and will give rise to neurectoderm and
the lens of the eye.
1) The resulting neuroectoderm will produce neuroblasts and certain
b) Ectodermal placodes can be grouped a:
1) one pair of Olfactory Placodes/Nasal Placodes
a] They form above the stomodeum.
b] They will become part of the lining of the nasal pits (that open into the
c] Some of these cells will develop into the neurosensory cells of the
2) one pair of Otic Placodes in the ectoderm lateral to the hindbrain
a] The ectoderm of the otic placodes will become the Otocysts as they
sink into the head.
1] The otocysts will give rise to the sensory epithelium of the internal
3) one pair of Optic Placodes/Lens Placodes which will develop into
the lens of the eye.
4) a series of Epibranchial Placodes on the side of the head that will
sink into the head along the midbrain and hindbrain.
a] These placodes will contribute neuroblasts that become cell
bodies in one pair or more of the ganglia of CN 7, 9, and 10.
1] Their nerve fibers will innervate the taste buds.
5) in fishes and amphibians a series of Linear Series of Placodes will
extend the length of he trunk and tail.
a] Some of these placodes will migrate into the head.
b] They will sink into the body to become the Neuromast Organs of
the cephalic and lateral lines.
6) Other placodes on the heads of fishes become electroreceptive
D, The Fate of the Endoderm
1. Endoderm will give rise to the epithelium of the entire digestive tube and its
2. Endoderm will also give rise to the epithelia of the parathyroids,
ultimobranchial glands, thymus, lining of eustachian tubes, and the lining of
the middle ear.
a) This endoderm derives from the pharyngeal pouches.
3. Midventral evaginations of the pharynx such as the thyroid, lungs, and
swim bladders, when present, are lined by endoderm.
4. Caudal to the pharynx the endoderm will evaginate to give rise to the liver,
gall bladder, pancreas, crop sacs, and a variety of gastric and intestinal ceca.
5. Since they are derived from the cloaca, mammalian urinary bladders and
urogenital sinuses are typically lined by endoderm.
E. The Fate of the Mesoderm
1. Dorsal Mesoderm or the Epimere
a) Most of the dorsal mesoderm will develop into block-like segments
running in pairs lateral to the notochord called Somites.
1) They run along the notochord and neural tube throughout the tail,
trunk, and, to a lesser degree, the head.
2) The somite will differentiate into three regions (running superficial to
a] Dermatome – gives rise to the dermis but only of the dorsal
1] The remainder of the dermis arises from the lateral plate
b] Myotome – gives rise to skeletal muscles.
c] Sclerotome – gives rise to the vertebrae, ribs, and portions of the
b) Some of the dorsal mesoderm located anterior to the somites are
arranged into incomplete segments called Somitomeres.
1) Somitomeres lack the dermatome and sclerotome components.
a] So somitomeres give rise to skeletal muscles.
1] These muscles are the extrinsic ocular muscles and the
2. Lateral Plate Mesoderm or Hypomere
a) Lateral plate mesoderm is confined to the trunk and divides into somatic
mesoderm and splanchnic mesoderm (which is separated by the coelom).
b) Somatic mesoderm will give rise to:
1) connective tissues of the body wall
2) blood vessels of the body wall
3) the skeleton of the body wall, limb girdles, and limbs
4) dermis of the body wall
5) and the parietal peritoneum.
c) Splanchnic mesoderm gives rise to:
1) smooth muscles and c.t.s of the digestive tube and its outpocketings
2) smooth muscles and c.t.s of the heart and visceral blood vessels
3. The Intermediate Mesoderm or Mesomere
a) Intermediate mesoderm is a pair of unsegmented mesodermal ribbons
running the length of the trunk between the lateral plate mesoderm and
b) It will give rise to kidney tubules and the longitudinal ducts of the
1) For this reason it is also known as “Nephrogenic Mesoderm”.
F. The Extraembryonic Membranes
1. Most vertebrates possess specialized membranes that extend beyond the
a) They arise early during development and provide a number of essential
b) The chief extraembryonic membranes are: yolk sac, amnion, chorion, and
1) The yolk sac is the most primitive and is founding all vertebrates.
2) The other three are found only in the amniotes (reptiles, birds, and
2. The Yolk Sac
a) The yolk sac surrounds the yolk and empties into the midgut.
1) It is lined by endoderm.
2) It is highly vascularized.
a] The associated blood vessels are termed Vitelline arteries, veins.
b] Vitelline vessels are confluent with blood vessels of the embryonic
3) As the embryo grows the yolk sac will diminish.
a] As it atrophies, the yolk sac will disappear into the ventral body wall.
1] The remaining intracoelomic yolk sac will either become
incorporated into the wall of the midgut or remain as an
b) Embryonic sharks develop a large yolk diverticulum (or outpocketing)
within the coelom.
1) This yolk diverticulum is still present when the shark is born and will
sustain it for several days.
c) In viviparous species of fish and amphibians the yolk sac, being highly
vascularized and close to internal tissues, will often serve as an oxygen
absorbing membrane to receive oxygen from the mother.
1) In this case it is sometimes referred to as a “Simple Yolk Sac
d) Therian mammals lack yolk but still will have a yolk sac.
1) It is called Meckelʼs Diverticulum.
2) It is a sign of our reptilian heritage.
3. The Amnion and Chorion
a) Amniotic embryos develop within two membranous sacs, the amniotic
and chorionic sacs.
1) Both the amnion and chorion form when upfoldings of the
somatopleure meet above the embryo and fuse together.
a] This will form the amniotic sac that will surround the embryo.
b] The somatopleure continues to extend and will grow around both
the amniotic sac and the yolk sac to give rise to the chorionic sac.
2) The evolution of these membranes allowed for the development of
the amniotic egg.
b) The amniotic sac surrounds the embryo and holds a fluid called
1) Amniotic fluid is mostly metabolic water from embryonic tissues.
2) It will protect the embryo from dehydration and mechanical trauma.
b) The chorion will lay in close association with either the eggshell or the
maternal body wall.
1) The chorion will keep the embryo supplied with oxygen either
through the eggshell or from the mother.
2) In viviparous species the chorion also supplies the embryo with
nourishment from the mother.
4. The Allantois
a) The allantois arises as a midventral evagination of the embryoʼs cloaca.
1) Normally it grows until it comes in contact with the chorion.
a] This forms the Chorioallantoic Membrane.
1] In monotremes and most reptiles the chorioallantoic membrane
serves for respiration.
2] In eutherian mammals it is in contact with the maternal lining and
makes up the embryonic portion of the placenta called the
b) The base o he placenta, that portion closest to the cloaca, becomes the
urinary bladder in amniotes.
c) In mammals a portion forms a portion of the umbilicus called the
Urachus or Middle Umbilical Ligament.
5. The Placenta
a) The placenta, in its broadest sense, is any region in a viviparous animal
where maternal and embryonic tissues of any type are in intimate contact
and allow for an exchange of materials.
1) This would include the simple yolk sac placenta of some nonamniotic
b) Most people consider a true placenta to exist only in amniotes.
1)] A true placenta would consist of:
a] a highly vascularized region of extraembryonic membrane such
as yolk sac, chorionic membrane, chorioallantoic membrane, or
b] associated highly vascularized maternal lining
2) Eutherian mammals have a chorioallantoic membrane.
3) In most marsupials the yolk sac, not the allantois, lays against the
chorion forming a Choriovitelline Placenta.
a] The extraembryonic membrane only lays closely against the
endometrium and can simply peel away from the endometrium
1] This is called a Contact or Nondeciduous Placenta.
4) Placental mammals have finger-like extensions called Chorionic
Villi that invade into the endometrium.
a] As a result during parturition a portion of the endometrium is lost.
1] This is termed a Deciduous Placenta.
b] The pattern of chorionic villi distribution varies between species.