Development of the First Day 135 



ting the age of chick embryos, during the first 

 two days. As has been said, the first somite 

 is formed at about the twenty-first or twenty- 

 second hour ; and at the end of the first day 

 five or six pairs are present. By the thirty- 

 sixth hour, there are about fifteen pairs of 

 somites : and by the end of the second day, 

 there are twenty-seven or twenty-eight pairs. 

 After the end of the second day the number 

 of somites can no longer be used to esti- 

 mate the age of embryos, though the somites 

 continue to increase in number until the 

 fourth day. The somites are formed in the 

 neck, trunk and tail regions, but do not ex- 

 tend into the head. 



In the first three or four somites, the cleav- 

 age of the mesoblast extends close up to the 

 notochord before the lateral plate is separated 

 from the vertebral, so that there is a cavity 

 in these somites that is continuous with the 

 body-cavity. This cavity is not seen in the 

 other somites, and eventually disappears from 

 these. 



The neurenteric canal, which was described, 

 in connection with the frog (page 34), as a nar- 

 row passage connecting the neural canal with 

 the extreme posterior end of the digestive 



