ONTOGENESIS 



209 



These begin development by an almost equal segmen- 

 tation resulting in the formation of a morula, composed 

 of cells of fairly uniform size. When the morula stage 

 has been completed, vacuoles begin to appear in some 

 of the central cells, become larger, coalesce, and give 

 origin to an irregular fissure-like space which is the 



'Elastbdertnic cavity. 

 -nvefo/xttf layer 



FIG. 92. Ovum of bat; differentiation of embryonic button. (Fan Beneden.) 



segmentation cavity, or lecithocele, and brings the ovum 

 to the stage of the blastodermic vesicle. Though not 

 formed in 'a manner identical with the regular blastulae 

 of the invertebrates, the blastodermic vesicle is easily 

 homologized with it and subserves the same purpose 

 as the blastula in the subsequent developmental stages. 



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 

 14 



