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 



hd 



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. pi, area pellucida; hd, head; med.gr, medullary 

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

 amnion; pr. st, primitive streak; pr. z/, 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 : 



i. The entoblast is formed by the rearrange- 



