48 



TEXT-BOOK OF EMBRYOLOGY. 



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 

 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. 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- 

 ficial layer of the cells of the morula. The result, then, is a single surface layer, 

 the covering layer, surrounding a central mass of polygonal cells (Fig. 33, a). 

 This solid mass of cells is transformed into a vesicle by vacuolization of some of 



Subzonal 



space 



Morula 



FIG. 32. Morula of rabbit, van Beneden. 



the inner cells (Fig. 33) and the confluence of these vacuoles to form a cavity. 

 The mammalian ovum at this stage thus consists of two groups of cells and a 

 cavity, an outer group or layer of cuboidal cells, the outer cell layer or covering 

 layer (trophoderm), forming the wall of the cavity, and an inner group of poly- 

 gonal or spheroidal cells, the inner cell mass which at one point is attached to 

 the outer layer of cells (Fig. 33, d). 



The mistake must not, however, be made of considering the mammalian 

 ovum at this stage as a true blastula. The mammalian ovum apparently does 

 not pass through any true blastula stage. Of the parts just described, the inner 

 cell mass alone is comparable to the blastoderm of birds, while the cavity cor- 

 responds not to the segmentation cavity but to the yolk mass of meroblastic 

 eggs. The vacuolization of the cells of the inner cell mass would thus repre- 

 sent a late and abortive attempt at yolk formation, the actual nutritive yolk 

 being made unnecessary, since the attachment of the ovum to the walls of the 

 uterus provides for direct parental nutrition. In the separation of the cells of 

 the morula into an inner cell mass and an outer covering layer is seen the earliest 



