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 



FIG. 46. DIAGRAMMATIC REPRESENTATIONS OF CHICK EMBRYOS; 

 A, AFTER TWENTY HOURS' INCUBATION; B, AFTER TWENTY-FOUR 

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



ar.op, area opaca; ar.pl, area pellucida; hd, head; tned.gr, medullary 

 groove; mes, mesoderm indicated by dotted outline and deeper shade; pr.am, pro- 

 amnion; pr.st, primitive streak; pr. v, proto-vertebrae. 



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 : 



i. The entoblast is formed by the rearrange- 



