I] THE FUOG. 159 



a. Tlie lozver layer ceils ; forming the lower half of 

 the whole; their boundary lines regular. 



ft. The 2ippej' layer cells ; forming the upper half, 



smaller and more numerous than those of the 



lower layer, their boundary lines in part irregular. 



i.e. the upper layer cells are dividing more 



rapidly than the lower ones. 



e. The same al 24. hours. 



llecognizable by the preponderance of the ])ig- 

 mented area. 



ct. The lower layer cells ; forming now tlie lower 

 third or thereabouts ; their boundary lines can 

 only be made out with difficulty. 



ft. The upper layer cells ; forming the upper two- 

 thirds at least ; their boundary lines no longer 

 clearly definable under a low power, the pig- 

 mented portion having a granulated appear- 

 ance. 



i.e. the upper layer cells are growing round 

 the lower ones. 



f. The sa?ne at 4 — 5 days. 



The lower layer cells apparent only as a small 

 A\]iite spot {blastopore or anus of RuscoJii). 



a. The blastojtieres ; boundary lines no longer 

 recognizable. 



ft. The blasiop07-e. Examine this with care ; its lip, 

 horse-shoe shaped, well defined above and at 

 the sides, ill-defined below. 



^'■. The same at G — 7 days. 



Signs of the embryo are now beginning to appear, 



«b 



