CLEAVAGE. 



a cavity, the segmentation cavity. The entire embryo is now known as the 

 blastula. 



The simplest type of blastula is seen in Amphioxus, where it consists of a 

 nearly spherical segmentation cavity surrounded by a single layer of cells. 

 Some of the cells those which are more ventral and contain the larger amount 

 of yolk are slightly larger than others (Fig. 26, 6). 



In the eggs of the frog, in which the cells resulting from segmentation show 

 greater inequality in size (due to difference in yolk content), the segmentation 

 cavity is surrounded by several layers of cells. In such a blastula the roof of 

 the cavity is comparatively thin, being composed of small cells containing little 

 yolk, micromeres, while the floor of the cavity is thick, being composed of large 



FIG. 31. Four stages in cleavage of the ovum of the mouse. Sobotta 

 Small cell marked with x is the polar body. 



:olk cells, macromeres. So thick is this wall of the vegetative pole of the blastula 

 that the large yolk cells extend into the segmentation cavity compressing it into 

 a crescentic cleft (Fig. 30). In the frog the roof of the segmentation cavity is 

 sharply denned from the floor, due to the fact that the outer layer of cuboidal 

 roof cells is densely pigmented. The rather sharply defined zone of transition 

 between pigmented micromeres and nonpigmented macromeres is known as the 

 marginal zone. 



In discoidal segmentation, the segmentation cavity is a mere slit between the 

 superficial protoplasmic cells and the underlying unsegmenting yolk with its 

 yolk nuclei (Fig. 29). Comparing it with unequal holoblastic cleavage, these 

 partially divided yolk cells which form the floor of the segmentation cleft in 

 discoidal cleavage are analogous to the large yolk cells which form the floor of 

 the segmentation cavity in the frog. (Compare Figs. 29 and 30.) 



