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 aortce; 

 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 eml:)ryonic 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 mesoblastic 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 lies 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 12 s 



Fig. 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 

 Figs. B, C, D, E. 

 a. V. e., Area vitellina externa, a. v. i.,Area vitellina interna. Bl. i., Blood 

 island. Bl. v., Blood vessel. Coel, Coelome. G. W., Germ-\yall. M. O., 

 Margin of overgrowth. N'ph., Nephrotome. S., Somite. Som'pl. Soma- 

 topleure. Sprpl., Splanchnopleure. Som. Mes., Somatic layer of mesoblast. 

 spl. Mes., splanchnic layer of the mesoblast. S. T., Sinus terminalis. Y. S. 

 Ent., Yolk-sac entoderm. Z. J., Zone of junction. 



