SEROUS AND MUCOUS LAYERS. 743 



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 Uastodermic or germinal membrane, as it 

 is called. Yery 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, 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. 



Yery 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 



