228 BIOLOGY: GENERAL AND MEDICAL 



formed in a manner identical with the regular blastulse 

 of the invertebrates, the blastodermic vesicle is easily 

 homologized with it and subserves the same purpose 

 as the blastula in the subsequent developmental stages. 



"Vau/f-of a/nniofi'c. , 



FIG. 93. Ovum of bat, showing amniotic cavity. X 200. (Van Beneden.) 



The mammalian ovum at this stage consists of a 

 layer of flattened cells, the " outer cell mass," surround- 

 ing a cavity, into which an irregular mass of cells of 

 more spherical form, "the inner cell mass," hangs sus- 

 pended from one side encroaching upon the space. 

 This embryo is imbedded in the uterine epithelium 



Vacuo/es m enibryomc iuffon. 



FIG. 94. Ovum of bat; differentiation of amniotic cavity. X 275. (Van 



Beneden.) 



which apparently melts away below its attachment in 

 order that placentation may be made possible later on. 



A free ovum in water or an ovum inclosed within a 

 shell is able to perfect its differentiations undisturbed 

 by contact with external bodies; but a mammalian ovum 

 of small size, without a nutrient yolk to feed upon and 

 situated in a crypt of the uterine mucosa, is obliged to 

 prepare for its future nutrition by effecting a communi- 

 cation with the maternal supply, and arrange for its 



