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TEXT-BOOK OF EMBRYOLOGY. 



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 

 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 



w 



FIG. 33. Four stages in the development of the bat. van Beneden. 



a, Section of morula; b, section of later stage of morula, showing differentiation of outer layer of 

 cells; c, section of still later stage, showing vacuolization of central cells; d, section showing outer 

 layer (trophoderm) and inner cell mass. 



polygonal 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 

 corresponds 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 represent 

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



