CHANGES OF THE OVUM. 581 



fully developed, they arrange themselves at the surface of 

 the yelk into a kind of membrane, and at the same time as- 

 sume 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 re- 

 mains 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 development of new cells, in- 

 creases 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 transpar- 

 ency than elsewhere. This space, the " area germinativa " 

 or germinal area, is the part at which the embryo first ap- 

 pears. 



At first the area germinativa has a rounded form, but it 

 soon loses this and becomes oval, then pear-shaped, and while 

 this change in form is taking place, there gradually appears 

 in its centre a clear space or area pellucida (Fig. 216), bounded 

 externally by a more opaque circle, the obscurity being due 

 to the greater accumulation of nucleated cells and nuclei at 

 that part than in the area pellucida. 



The first trace of the embryo in the centre of the area pellu- 

 cida consists of a shallow groove or channel, the primitive 

 groove (Fig. 216), formed of the external or serous fold of the 



49 



