64 EARLIEST CHANGES IN THE OVUM. 



small clear vesicles, exactly similar to those mentioned as being formed 

 within the germinal vesicle, were observed scattered through the yolk. It 

 appears, therefore, that the fate of the germinal vesicle with its germinal 

 spot, is still matter of uncertainty. 



Changes in the tunica granulosa. Both Barry and Bischoff have observed, 

 that the cells of the membrana granulosa of the ovisac, which immediately 

 surround and adhere to the ovum, undergo a peculiar change of form about 

 the time at which the ovum is destined to leave the ovary. They become 

 club-shaped, their pointed extremities being attached to the zona pellucida, 

 so as to give the ovum a stellate appearance (see fig. 5). The club- 



Fig. 5.* 



shaped extremity of each cell contains a distinct nucleus, and Barry states, 

 that in place of this nucleus, a pellucid space is afterwards seen ; that a 

 young cell succeeds, and that subsequently, the whole cell becomes filled 

 with other cells. Bischoff, however, has seen none of the latter appearances. 

 But he has observed, that when the ovum enters the Fallopian tube, these 

 cells lose their spindle-or club-like shape, and become quite round. In the 

 bitch, they continue to invest the ovum in this round shape throughout 

 the whole tract of the Fallopian tube disappearing only when the ovum 

 reaches the uterus \ but in the rabbit, they wholly disappear at its very 

 commencement. 



Contraction of the Yolk. Formation of the Chorion. Besides the disap- 

 pearance of the germinal vesicle, and, in the rabbit, the disappearance also 

 of the cells of the membrana granulosa, it is observed, according to BischofY, 

 that in the upper part of the Fallopian tube, the yolk no longer completely 

 fills the zona pellucida, but that a clear fluid collects between them, and 

 that the contour of the yolk becomes defined by a dark line. This change 

 Bischoff ascribes to a contracted and consequently more dense condition of 

 the yolk, the granules composing which now adhere together so firmly, 

 that when the yolk is broken down with a needle, they do not become dif- 

 fused through the surrounding fluid, as they would have done previously. 



* Fig. 5. A. An ovarian ovum from a bitch in heat, exhibiting the elongated form and 

 stellate arrangement of the cells of the discus proligerus or membraua granulosa around the 

 zona pellucida. B. The same ovum after the removal of most of the club-shaped cells. 



t Entwickcl. des Hunde-eies, p. 41. 



