742 GENERATION AND DEVELOPMENT. 



globule : these granules usually arrange themselves con- 

 centrically around the nucleus. When the peripheral cells, 

 which are formed first, are fully developed, they arrange 

 themselves at the surface of the yelk into a kind of mem- 

 brane, and at the same time assume a pentagonal or 

 hexagonal shape from mutual pressure, so as to resemble 

 pavement-epithelium. As the globular masses of the 

 interior are gradually converted into cells, they also pass 

 to the surface and accumulate there, thus increasing the 

 thickness of the membrane already formed by the more 

 superficial layer of cells, while the central part of the yelk 

 remains filled only with a clear fluid. By this means the 

 yelk is shortly converted into a kind of secondary vesicle, 

 the walls of which are composed externally of the original 

 vitelline membrane, and within by the newly-formed 

 cellular layer, the blastodermic or germinal membrane, as it 

 is called. Very soon, however, the latter, by the develop- 

 ment of new cells, increases in thickness, and splits into 

 two layers, so that now the ovum has three coats. The 

 vitelline membrane on the outside, and, within this, the 

 outer and the inner layers of the blastodermic membrane. 



Of the last-named layers, the superior or outer, which 

 lies next to the zona pellucida or vitelline membrane, is 

 called the serous layer ; from it are developed the organs 

 of the animal system of the body, e.g., the bones, muscles, 

 and integuments. The inferior or inner layer, in contact 

 with the yelk itself, is named the mucous layer, and serves 

 for the formation of the internal or visceral system of 

 organs. 



Changes of the Ovum within the Uterus. 



Very soon after its formation, and division into two 

 layers, the blastodermic vesicle or membrane presents at 

 one point on its surface an opaque roundish spot, which is 

 produced by an accumulation of cells and nuclei of cells, 

 of less transparency than elsewhere. This space, the 



