24-Hour and 33-Hour Chick Embryos
24-HOUR CHICK 2. Splanchnic Mesoderm [SP]:
mesodermal layer in contact with
I. Whole Mount endoderm.
A. Area Pellucida [APL]: relatively clear 3. Somatic Mesoderm [SO]: meso-
area immediately surrounding embryo. dermal layer in contact with
B. Area Opaca [AO]: outside area pellu- ectoderm.
cida with somewhat mottle appear-
ance. Splanchnic and somatic mesoderms are best
C. Neural Folds [NF]: two dense lateral seen in embryo sections between subcephalic
bands running parallel to each other pocket and somites as part of lateral body
throughout middle of embryo; space folds.
between these is neural groove; if
neural folds have fused cranially, a C. Endoderm [EN]: inner layer of embryo
neural tube is present (may not have (seen in later sections).
occurred in some 24-hour embryos). D. Neural Tube [NT]: formed by fusion or
D. Somites: paired blocks of mesoderm meeting of neural folds.
lateral to neural folds. E. Neural Plate [ONP]: thickened tissue
E. Notochord [NO]: here, seen between seen on surface of embryo; found dor-
neural folds by focusing up and down; sal to notochord.
a narrow mesodermal rod which sup- F. Neural Groove [ONP]: space between
ports the developing embryo. sides of neural plate (neural folds);
F. Primitive Streak [PS]: continues caud- becomes lumen of neural tube.
ally from notochord. G. Notochord [NO]: seen throughout em-
bryo sections; usually a round mass of
II. Serial Transverse Sections tissue in very middle of embryo,
A. Ectoderm [E]: outer (or upper) tissue ventral to neural tube or plate.
of embryo. H. Foregut [FG]: flattened space (dorso-
1. Epidermal Ectoderm [EE]: outer- ventrally) ventral to neural tube and
most layer of tissue covering notochord
embryo. J. Anterior Intestinal Portal [AIP]: found
2. Neural Ectoderm : tissue making caudal to foregut; an opening through,
up neural tube or neural folds. what appears to be, ventral side of
B. Mesoderm [M]: middle layer of tissue embryo.
1. Somite Mesoderm [SM]: tissue K. Heart Primordium [H]: distinct thick-
making up a somite (seen in later ening of splanchnic mesoderm on
sections). either side of midline at about level of
anterior intestinal portal
33-HOUR CHICK C.-G. Continuing caudally oval or pear shaped
midbrain [MB] appears, just posterior to
I. Whole Mount infundibulum; moving further back hind-
A. Area Opaca [AO]: two parts: brain [HB] can be seen (at about level of
1. Area Vasculosa [VAS]: inner heart ventricle); note change in shape of
mottled area caused by neural tube; spinal cord [NT] is seen where
developing blood islands [BI]. lateral walls become much thicker than
2. Area Vitellina [VIT]: non-mottled dorsal or ventral walls; as neural tube
area covering the yolk. opens (and is no longer a tube) we are in
Areas Vasculosa and Vitellina are area of neural fold [NF] and neural groove
separated from each other by [ONP].
sinus terminalis, a circular blood H. Notochord [NO]: initially seen a few
vessel. sections back from infundibulum, seen
B. Area Pellucida [APL] as small mass of cells beneath neural
C. Anterior Neuropore [ANP]: opening at tube.
most cranial end of neural tube. I. Foregut Lumen [FG]: beginning at level
D. Forebrain (or prosencephalon) [FB]: of optic vesicles, trace caudally to fol-
cranial most enlargement of low a small endoderm lined foregut,
embryonic brain; most characterized found ventral to neural tube; in poster-
by presence of optic vesicles, ior sections foregut widens and it’s
beginnings of eyes. floor makes contact with ventral skin
E. Midbrain (or mesencephalon) [MB]: ectoderm, forming oral membrane
division of brain caudal to forebrain. [OM].
F. Hindbrain (rhombencephalon) [HB]: J. Dorsal Aorta [DA]: two spaces seen at
caudal most section of embryonic level of midbrain between midbrain
brain; may appear as two or three and foregut.
bulges of neural tube. K. Ventricle of the Heart [V]: approxi-
G. Neural Tube or Spinal Cord [NT]: mately at level of oral membrane will
section of neural tube caudal to hind- be seen an enlargement that bends
brain; beyond this, tube diverges as from midline somewhat towards em-
neural folds and continues as open bryo’s left; notice that endocardium
neural plate; continuing yet further as [END] is separated from epimyocard-
primitive streak. ium [EMY].
H. Notochord [NO]: best seen in region L. Sinus Venosus [SV]: in more caudal
of brain. sections heart “moves” to midline and
endocardium and epimyocardium are
II. Serial Transverse Sections no longer as separated; region where
A. Forebrain [FB]: broad anterior section sinus venosus is found and where
of brain containing lateral optic atrium will form.
vesicles [OP]. M. Vitelline Vein [VV]: caudal to sinus
B. Infundibulum [IN]: outpocketing of venosus as two lateral vessels
floor of forebrain seen as optic consisting only of endothelium.
vesicles fade out while moving
Legend of Structures for 24 Hour and 33 Hour Chicks
A1 first aortic arch HB hindbrain, rhomben- PCV posterior cardinal vein
AC anterior cardinal vein cephalon PE pericardial coelom
3+ H heart primordium PF primitive fold
0AIP anterior intestinal portal HM head mesoderm PG primitive groove
AMM anterior margin of invagin- HN Hensen’s node PP primitive pit
ated lateral mesoderm IC internal carotid artery PS primitive streak
ANP anterior neuropore IN infundibulum SCP subcephalic pocket
AO area opaca LBF lateral body fold SI sinus terminalis (circular
APL area pellucida LM lateral plate (hypomere) vein of yolk sac)
AT atrium of heart mesoderm SM somite (epimere) mesoderm
BI blood islands M mesoderm SO somatic lateral plate (hypo-
CA conus arteriosus MB midbrain, mesencephalon mere) mesoderm
CAP capillaries MFH margin of free head SOP somatopleure
CC common cardinal vein MG midgut open to yolk SP splanchnic lateral plate (hy-
CL coelom MM mesomere, nephrotome, Pomere) mesoderm
CNC cranial neural crest primor- intermediate mesoderm SPP splanchnopleure
dium of nerves 5, 7, 8 MYC myocoele of somite SV sinus venosus
DA dorsal aorta NC neural crest TM tailbud mesoderm
DMC dorsal mesocardium NCL neurocoele, neural canal USM unsegmented somite meso-
E ectoderm NF neural fold Derm
EE epidermal ectoderm NM neuromeres of hindbrain V ventricle of heart
EMB ambryonal areal NO notochord VA ventral aorta
EMY epimyocardial layer of heart NT neural tube VAS area vasculosa
EN endoderm OM oral membrane VIT area vitelline
END endoderm ONP open neural plate VV vitelline vein
FB forebrain, prosencephalon OP optic vesicle Y yolk
FG foregut lumen OT otic placode YS yolk sac
FGE foregut endoderm PA proamnion ZJ zone of junction