136 Vertebrate Embryology 



tract, is seen in the early stages of the chick 

 embryo as two or three small depressions 

 in the floor of the posterior end of the neu- 

 ral canal. These depressions usually appear, 

 one after the other, during the first three days, 

 and, though they seldom are seen to open 



''^^iy 



pr.i^ 



- prst 



Fig. 46. — Diagrammatic representations of chick embryos; 

 a, after twenty hours' incubation; b, al'ter twenty-four 

 hours' INCUBATION. (After Parker and Haswell, from Marshall.) 



ar.op^ area opaca; ar.pt, area pellucida; hd, head; ined.^r, medullary 

 groove; fitts, mesoderm Indicated by dotted outline and deeper shade; pr.am, pro- 

 amnion; pr. si, primitive streak; pr. v, proto-vertebrse. 



into the digestive tract, they seem to be homo- 

 logous with the neurenteric canal of the frog 

 and of other animals. 



We shall now briefly summarize the more im- 

 portant changes that take place during the 

 first day : 



1. The entoblast is formed by the rearrange- 



