I.] THE FROG. 159 



a. The lower layer cells ; forming the lower half of 

 the whole, their boundary lines regular. 



fi. The upper 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 at 24 hours. 



Recognizable by the preponderance of the pig- 

 mented area. 



a. The lower layer cells ; forming now the lower 

 third or thereabouts ; their boundary lines can 

 only be made out with difficulty. 



y8. The tipper 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 same at 4 5 days. 



The lower layer cells apparent only as a small 

 white spot (blastopore']. 



a. The blastomeres ; boundary lines no longer 



recognizable. 

 ft. The blastopore. Examine this with care ; its lip, 



horse-shoe shaped, well defined above and at 



the sides, ill-defined below. 



g. The same at 6 7 days. 



Signs of the embryo are now beginning to appear, 



