EMBRYO WITH TWENTY-EIGHT SEGMENTS. 



279 



together with a portion of the ectoderm, a part of the true amnion, Am' , of the 

 head. The entoderm, Ent, on the contrary, bends to the left and joins the ecto- 

 derm on that side of the head to form the pro-amnion, Pro. am. Beyond the 

 head the entoderm and mesoderm asjain unite and we have a continuation of the 



cl.III. 



Pro. am. 



Fig. 157. — Section of Chick Embryo with about Twenty-eight Segments. Transverse Series 92, 



Section 114. 



Am, Am', Amnion. Ao.D, Descending aorta. A0.2, Second aortic arch. Cho, Chorion. cl.II, Second ento- 

 dermal gill pouch. cl.III, Third entodermal gill pouch. El, Ectoderm. En, Entoderm of pharynx. 

 endo, Endothelial heart. Ent, Entoderm of proamnion. /./>, Fore-brain. Md, Medulla oblongata, mes, 

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

 amnion. Seg, Segment. Spl, Splanchnopleure. Ve, Anterior cardinal vein. X 5° diams. 



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

 tions of the foetal envelopes surrounding the head are complicated. The student 

 may, however, easily satisfy himself that the layer, Ec, in figure 157, is really 



