So 



THE STUDY OF CHICK EMBRYOS 



neural tube in the region of the primitive segments. The primitive streak is 

 still prominent at the posterior end of the area pellucida, forming about one-fourth 

 the length of the embryo. Transverse sections through the primitive streak 



and open neural groove show approximately the 

 same conditions as in the twenty-hour embryo 

 (Figs. 26 and 28). 



A Transverse Section through the Fifth 

 Primitive Segment (Fig. 33) is characterized by 

 the differentiation of the mesoderm, the approxi- 

 mation of the neural folds and the presence of two 

 vessels, the descending aortas on each side between 

 the mesodermal segments and the entoderm. The 

 neural folds are thick and the ectoderm is thickened 

 over the embryo. The notochord is a sharply de- 

 fined oval mass of cells. The mesodermal segments 

 are somewhat triangular in outline and connected 

 by the intermediate cell mass with the lateral meso- 

 derm. This is partially divided by irregular flat- 

 tened spaces into two layers, the dorsal of which is 

 the sflmaticy the ventral the splanchnic layer of 

 megoderm. Later, the spaces unite on either side 

 to form the ccelojnoi^ri mitive body cavity. 



Transverse Section Caudal to the Fovea 

 Cardiaca (Fig. 34). — The section is characterized 

 (1) by the closing together of the neural folds to 

 form the neural tube; (2) by the dorsad and laterad 

 folding of the entoderm which, a few sections nearer 

 the head end, forms the fore-gut or pharynx; (3) 

 by the presence of the vitelline veins laterally be- 

 tween the entoderm and mesothelium; (4) by the 

 wide separation of the somatic and splanchnic 

 mesoderm and the consequent increase in the 

 size of the ccelom. In this region, it later sur- 

 rounds the heart and forms the pleuro-pericardial 

 cavity. 

 The neural tube in this region forms the third brain vesicle or hind-brain. 

 The neural folds have not yet fused. Mesodermal segments do not develop in 



Fig. 32. — Median longitu- 

 dinal section of a thirty-six-hour 

 chick embryo (Marshall). AN, 

 amnion fold; BF, fore-brain; BR, 

 hind-brain; BM, mid-brain; CH, 

 notochord; CP, pericardial cav- 

 ity; GF, fore-gut; H, entoderm; 

 NS, spinal cord; NT, neurenteric 

 canal; PS, primitive streak; RV, 

 ventricle of heart; SO, somato- 

 pleure; SP, splanchnopleure; 

 TA, allantois. 



