THE STAGES OF EARLY
From egg to organism, an animal’s form develops
gradually: the concept of epigenesis
• An embryo is not preformed in an egg; it
develops by epigenesis, the gradual,
gene-directed acquisition of form.
Fertilization activates the egg and brings
together the nuclei of sperm and egg
• Fertilization both reinstates diploidy and
activates the egg to begin a chain of metabolic
reactions that triggers the onset of embryonic
• The acrosomal reaction, which occurs when the
sperm meets the egg, releases hydrolytic
enzymes that digest through material
surrounding the egg.
• Gamete fusion depolarizes the egg cell
membrane and sets up a fast block to
• Sperm-egg fusion also initiates the cortical
reaction, involving a signal-transduction pathway
in which calcium ions stimulate cortical granules
to erect a fertilization envelope that functions as
a slow block to polyspermy.
• In mammalian fertilization, the cortical reaction
hardens the zona pellucida as a slow block to
• SEE Figure 47.5 on page 1002
Cleavage partitions the zygote into
many smaller cells
• Fertilization is followed by cleavage, a period of rapid cell
division without growth, which results in the production of
a large number of cells called blastomeres.
• Holoblastic cleavage, or division of the entire egg, occurs
in species whose eggs have little or moderate amounts
• Meroblastic cleavage, incomplete division of the egg,
occurs in species with yolk-rich eggs.
• Cleavage planes usually follow a specific pattern relative
to the animal and vegetal poles of the zygote.
• In many species, cleavage creates a multicellular ball
called the blastula, which contains a fluid-filled cavity, the
• SEE FIGURE 47.8
Sea Urchin Development Video
Gastrulation rearranges the blastula to form
a three-layered embryo with a primitive gut
• Gastrulation transforms the blastula into a
gastrula, which has a rudimentary
digestive cavity (the archenteron) and
three embryonic germ layers: the
ectoderm, endoderm, and mesoderm.
• SEE FIGURE47.10 p.1006
In organogenesis, the organs of the
animal body form from the three
• Early events in organogenesis in
vertebrates include formation of the
notochord by condensation of dorsal
mesoderm, development of the neural
tube from folding of the ectodermal neural
plate, and formation of the coelom from
splitting of lateral mesoderm.
• SEE FIGURE 47.11, p. 1008
Frog Development Video
Amniote embryos develop in a fluid-
filled sac within a shell or uterus
• Meroblastic cleavage in the yolk-rich, shelled
eggs of birds and reptiles is restricted to a small
disc of cytoplasm at the animal pole.
• A cap of cells called the blastodisc forms and
begins gastrulation with the formation of the
• In addition to the embryo, the three germ layers
give rise to the four extraembryonic membranes:
the yolk sac, amnion, chorion, and allantois.
• The eggs of placental mammals are small and
store little food, exhibiting holoblastic cleavage
with no obvious polarity.
• Gastrulation and organogenesis, however,
resemble the processes in birds and reptiles.
• After fertilization and early cleavage in the
oviduct, the blastocyst implants in the uterus.
• The trophoblast initiates formation of the fetal
portion of the placenta, and the embryo proper
develops from a single layer of cells, the
epiblast, within the blastocyst.
• Extraembryonic membranes homologous to
those of birds and reptiles function in intrauterine
THE CELLULAR AND
MOLECULAR BASIS OF
DIFFERENTIATION IN ANIMALS
Morphogenesis in animals involves
specific changes in cell shape,
position, and adhesion
• SEE Figure 47.16 and 47.17, p. 1012-1014
• Cytoskeletal rearrangements are responsible for
changes in both shape and position of cells.
• Both kinds of changes are involved in tissue
invaginations, as occurs in gastrulation, for example.
• The extracellular matrix provides anchorage for cells and
also helps guide migrating cells toward their
• Cell adhesion molecules on cell surfaces are also
important for cell migration and for holding cells together
The developmental fate of cells depends on
cytoplasmic determinants and cell-cell
Fate mapping can reveal cell
genealogies in chordate embryos
• SEE Figure 47.20, pp. 1014-1015
• Experimentally derived fate maps of
embryos have shown that specific regions
of the zygote or blastula develop into
specific parts of older embryos.
The eggs of most vertebrates have
cytoplasmic determinants that help establish
the body axes and differences among cells
of the early embryo
• SEE FIGURE 47.21, p. 1015
• When cytoplasmic determinants are
heterogeneously distributed in an egg,
they serve as the basis for setting up
differences among parts of the egg and,
later, among the blastomeres resulting
from cleavage of the zygote.
• Cells that receive different cytoplasmic
determinants acquire different fates.
Inductive signals drive
differentiation and pattern formation
• SEE FIGURES 47.22-47.24, pp. 1016-1019
• Cells in a developing embryo receive and interpret
positional information that varies with location.
• This information is often in the form of signal molecules
secreted by cells in special "organizer" regions of the
embryo, such as the dorsal lip of the blastopore in the
amphibian gastrula and the apical ectodermal ridge of
the vertebrate limb bud.
• The signal molecules influence gene expression in the
cells that receive them, leading to differentiation and the
development of particular structures.