242 OUTLINES OF CHORDATE DEVELOPMENT 



time, twenty-one to twenty-seven hours after ovulation, gas- 

 trulation has begun and the egg is ready to be laid. If the 

 completely formed egg reaches the vagina during the middle of 

 the day (8 A. M. to 4 P. M.) it may be laid promptly; otherwise 

 it is retained within the vagina until the following day. In the 

 latter case, since development continues during the entire 

 period of its retention, which may be quite prolonged, the 

 embryo may be in a fairly advanced stage when the egg is 

 finally deposited. Thus, variation in the period of retention 

 accounts for the variation in developmental stages observed in 

 different eggs as laid. 



TABLE SHOWING THE CHIEF EVENTS IN THE EARLY 

 HISTORY OF THE HEN'S EGG 



We may now return to describe the processes of development 

 occurring during the period prior to laying. The unicellular 

 germ disc consists of a quite definitely circumscribed area at 

 the animal pole of the vitellus. The disc is about 3 mm. in 

 diameter and less than 0.5 mm. in thickness. Beneath, the 



