204 



STUDY OF YOUNG CHICK EMBRYOS. 



entoderm, Ent, on the contrary continues in the same direction as before, until it 

 joins the ectoderm on the left side of the head to form the pro-amnion, Pro.am. 

 Beyond the head the entoderm and mesoderm again unite and we have a continua- 

 tion of the splanchnopleure, Spl. Owing to the development of the pro-amnion, 

 the relations of the fetal envelopes surrounding the head are complicated. The 



d.III 



FIG. 155. SECTION OF CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, 



SECTION 114. 



Am, Am', Amnion. Ao.D, Descending aorta. Ao.2, Second aortic arch of the left side. Cho, Chorion. cl.II, 

 Second entodermal gill-pouch. cl.III, Third entodermal gill-pouch. EC, Ectoderm. En, Entoderm of 

 pharynx, endo, Endothelial heart. Ent, Entoderm of pro-amnion. f.b, Fore-brain. Md, Medulla oblongata. 

 mes, Mesoderm of amnion. m.ht, Muscular heart, nch, Notochord. Pro.am, Pro-amnion. raph, Raphe of 

 amnion. Seg, Segment. Spl, Splanchnopleure. Ve, Anterior cardinal vein. X 50 diams. 



student may, however, easily satisfy himself that the layer, EC, in figure 157, is 

 really ectoderm by following it through in the series of sections, for he will then 

 find that it becomes continuous in other regions, on the one hand, with the ecto- 

 derm of the true amnion, and, on the other, with the epidermis of the body proper. 

 In the cervical region we have a transverse section of the lower portion of the 





