40 



DEVELOPMENT OF THE FROG'S K(;(i 



[Ch. IV 



pole. The division of the lower eight blastomeres is sometimes 

 so regular that a circle of eight dark cells is formed around the 

 equator of the egg (Fig. 12, C). After this division thirty- 

 two cells are present. At this period the distribution of the 

 cells over the dark and light regions of the egg is such that 

 the cells on the side of the egg showing the light crescent above 

 are smaller than the corresponding cells on the opposite side. 



Fig. 15. — Stages in the segmentation (A, B, C) of the egg, and the beginning of 

 gastrulation (D). AR. Beginning of archenteron. SG. Segmentation-cavity. 



Up to the present time all the divisions of the cells started 

 on the outside of the egg or blastomeres and progressed inward. 

 As early as the eight-cell stage, a cavity appeared between the 

 cells in the upper hemisphere of the egg. This is the "seg- 

 mentation-cavity." It is filled with an albuminous fluid which 



