THE EARLY DEVELOPMENT OF THE MAMMAL 383 



egg axis, but the promorphological relations of these cleavages 

 are not known. 



In a comparatively early stage the cells take on a fairly 

 definite arrangement. Thus on the surface of the cleavage 

 group we may see a regular and usually continuous, epithelium- 

 like layer, surrounding, or nearly surrounding, a central mass 

 of large, irregularly arranged cells (Fig. 148). The egg 

 usually remains closely surrounded by the zona pellucida dur- 

 ing these stages and the superficial cell layer is known as the 

 subzonal layer, the central mass as the inner cell mass. 

 This arrangement of cells forms what is known as the morula 

 stage, equivalent to the blastula stage of other forms. (The 

 identification of this stage as a gastrula ("metagastrula," 

 Van Beneden) leads to some difficulties in the interpretation of 

 the homologies of the layers of later stages.) The fully formed 

 morula consists of thirty-six to seventy-two cells, of which 

 twenty-four to thirty compose the inner cell mass. 



3. The Blastodermic Vesicle 



As cleavage continues in the cells of the morula, vacuoles 

 appear among the cells of the inner cell mass, toward one side 

 of the morula only. These vacuoles rapidly enlarge and flow 

 together toward one pole, while the cells of the inner mass re- 

 main grouped together at the opposite pole (Fig. 148). The 

 structure thus formed is termed the blastodermic vesicle; the 

 fluid of the cavity is supposed to represent the yolk-mass of the 

 sauropsid blastula or gastrula. 



In all cases the uterus is reached by the time the blasto- 

 dermic vesicle is formed, and the immediately subsequent 

 events in development may be described under two heads, 

 (a) the growth or enlargement of the blastodermic vesicle; 

 (6) the formation of the embryonic rudiment and its layers 

 (germ layers). The implantation or embedding of the vesicle 

 in the uterine wall will be described in connection with the 

 history of the foetal membranes. Although these processes 

 overlap to a considerable extent we may describe them sepa- 

 rately, as a matter of convenience. 



