124 THE DEVELOPMENT OF THE CHICK 
pleure, from which arises the intestine and all its appendages, 
including the allantois and the yolk-sac. Blood-vessels lie be- 
tween the splanchnic mesoblast and the entoderm. The large 
vessels beneath the somite and nephrotome are the dorsal aorte; 
small vessels are present in the area pellucida, and there are 
many large ones in the area vasculosa. The walls of the vessels 
are constituted of a single layer of flat endothelial cells bulging 
in the region of the nuclei; in the vascular area are true blood- 
islands with embryonic blood-cells more or less fully filling the 
cavity. 
In a median sagittal section (Fig. 67) the following points 
should be noticed: (1) the neural tube is enlarged in the region of 
the head to form the brain, more fully described below; (2) the 
entoderm forms a tube in the head known as the pharynx or 
cephalic enteron (cephalic part of the fore-gut), opening behind 
the heart into the space between the entoderm and yolk. The 
floor of the anterior end of the fore-gut is fused to the ectoderm 
in the middle line forming the oral plate. The entoderm forming 
the floor of the fore-gut turns forward around the hind end of 
the heart, and beneath the anterior part of the head forms part 
of the proamnion or mesoderm-free region of the pellucid area; 
(3) the large pericardial (parietal) cavity lies beneath the floor 
of the fore-gut. Attached to the posterior wall of the pericar- 
dium one sees the hind end of the heart with its two walls, the 
endocardium and the myocardium a fold of the mesoblastie lin- 
ing of the pericardium. Between the anterior end of the pericar- 
dium and the oral plate is seen the endothelial ventral aorta; (4) 
the notochord les between the fore-gut and neural tube and ends 
anteriorly in a mass of mesenchyme lying between the infundib- 
ulum and fore-gut. 
The Nervous System. The neural tube is closed at the 12s 
Fic. 68. — A. Transverse section across the axis of the embryo and the en- 
tire blastoderm of one side. The section passes through the sixth somite 
of a 10 s embryo, and is intended to show the topography of the blastoderm. 
The regions B, C, D, E are represented under higher magnification in the 
Hiss: B,C, Dy E. 
a.v.e., Area vitellina externa. a.v.i,Area vitellina interna. BI. i., Blood 
island. Bl. v., Blood vessel. Coel., Coelome. G. W., Germ-wall. M. O., 
Margin of overgrowth. N’ph., Nephrotome. $., Somite. Som’pl. Soma- 
topleure. Spl’pl.,Splanchnopleure. Som. Mes., Somatie layer of mesoblast. 
spl. Mes., splanchnic layer of the mesoblast. 5S. T., Sinus terminalis. Y. 5. 
Ent., Yolk-sac entoderm. Z. J., Zone of junction. 
