CLEAVAGE. 



51 



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.) 



In the mammalian ovum, as in the other cases just described, segmentation 

 leads up to the formation of a solid mass of cells — the morula. While cleavage 



Fig. 31. — Four stages in cleavage of the ovum of the mouse. Sobotta 

 Small cell marked with x is the polar body. 



here is of the holoblastic equal type, the irregularity is especially marked. In 

 the mouse, for example, the second cleavage is complete in one of the blasto- 

 meres before it has begun in the other, so that a three-celled stage results 



Fig. 32. — Morula of rabbit, van Beneden. 



(Fig. 31). Following this is a four-celled stage. From this time on cleavage 

 continues irregularly until a solid mass is formed, as in the lower forms, 

 which is composed of apparently similar cells (Fig. 32). 



The next step in mammalian development is a differentiation of the super- 



