574 



GENERATION. 



[CHAP. XL. 



Formation of Blastodermic Vesicle, or Germinal Membrane. 

 About the time that the ovum reaches the uterus, the segmenta- 

 tion is complete, and in its general appearance it much resembles 

 the ovarian ovum ; but according to Bischoff, upon careful exami- 

 nation, it is found that the apparently granular mass is really 

 aggregated into minute spherical masses, in the centre of each of 

 which is situated a clear vesicle. Soon each of these collections of 

 granules become surrounded with an investing membrane, so that 

 cells are formed, the nuclei of which are represented by the clear 

 central vesicle. The more peripheral cells are first formed, and 

 these from mutual pressure assume a pentagonal or hexagonal 

 form ; they become flattened, and united together at their margins, 

 like pavement epithelium. The same process takes place in the 

 interior of the yolk-mass, and as the cells are formed they pass 

 towards the surface, and thus the thickness of the layer first pro- 

 duced becomes increased. A clear fluid only occupies the central part 

 of the yolk. Thus, after the termination of the cleavage process, 

 a membrane composed of cells is formed within the zona pellucida, 

 which Bischoff has termed the " blastodermic vesicle." Soon after 

 the formation of this membrane, but not until it has increased in 

 thickness by the addition of new cells upon its inner surface, 

 formed from the contained yolk-mass, an opaque roundish spot, 

 consisting of cells and nuclei, makes its appearance at one spot. 



Fig. 27 1 



Ovum from the ovary of the Guinea-pig, surrounded by the 

 membrana granulosa, through which the zona pellucida is distin- 

 guished. 



