SEGMENTATION OF THE EGG. 103 



At the stage represented by Figs, 47 and 48, when eight 

 cells are present, i.e. on the completion of the third cleft, a 

 small cavity appears in the centre of the egg, between the inner 

 ends of the cells (Fig. 48). This is the segmentation cavity or 

 blastocoel. From its first appearance it is situated nearer the 

 upper than the lower pole of the egg. It is filled with fluid, 

 and during the later phases of segmentation it increases con- 

 siderably in size (Figs. 49, 50). 



At the close of segmentation the egg has the structure shown 

 in section in Fig. 50. It is a hollow ball, the same size as the 

 original ovum, with a small, excentrically-placed cavity, and 

 with walls of very unequal thickness. The cells of the upper half 



FIG. 49. FIG. 50. 



FIG. 49. The blastula stage in the development of the Frog's Egg, bisected 



vertically, x 20. 

 FIG. 50. The Frog's Egg at the close of segmentation, bisected vertically. 



x20. 



B, segmentation cavity or blastocoel. 



are small, approximately uniform in size, and arranged more or 

 less definitely in two layers, outer and inner ; while the cells of 

 the lower half are larger, and much more irregular in shape, 

 size, and arrangement : furthermore, the superficial cells of the 

 upper half are deeply pigmented at their outer ends, while those 

 of the lower half are nearly colourless. 



The distinction between upper and lower cells is, however, 

 not an absolute one, the cells at the equator being intermediate 

 in all respects between those of the upper and lower poles. 



The stage represented in Fig. 49 is the one which corresponds 

 most closely with the blastula stage of Amphioxus (Fig. 14, vm). 

 There are, however, important differences between the two. In 

 the blastula stage of the frog there are fewer component cells ; 



