38 THE UNIMPREGNATED OVUM. 



production of round bodies resembling the germinal spots of ova, the ger- 

 minal vesicles being subsequently developed around these in the form of 

 transparent membranous cells. 



The more important changes that take place in the ovum subsequent to 

 the formation of its essential component parts consist in alterations of the 

 size and position of those parts with relation to each other, and of the 'ovum 

 itself with relation to the Graafian follicle, and in the more complete 

 elaboration of the yolk. 



The earlier the stage of development the larger is the germinal vesicle 

 in relation to the whole ovum, and the ovum in relation to the Graafian 

 follicle. For, as the ovum becomes mature, although all these parts 

 increase in size, the Graafian follicle enlarges most, and the germinal 

 vesicle least. Changes take place also in the position of the parts. The 

 ovum at first occupies the centre of the Graafian follicle, but subsequently 

 is removed to its periphery. The germinal vesicle, too, which in young 

 ova is in the centre of the yolk is in mature ova found at the periphery.* 



The change of position of the ovum from the centre to the periphery 

 of the Graafian follicle is probably connected with the formation of the 

 membrana granulosa which lines the follicle. For, according to Valen- 

 tin, f at a very early period the contents of the follicle between its wall 

 and the ovum is almost wholly formed of granules, but in the process of 

 growth a clear fluid collects in the centre of the follicle and the granules 

 which from the first have a regular arrangement are pushed outwards, and 

 form the membrana granulosa. Now as the mature ovum lies imbedded 

 in a thickened portion of the membrana granulosa, it seems probable that 

 when the elementary parts of this membrane are pushed outwards in the 

 way just described, the ovum is carried with them from the centre to the 

 periphery of the follicle. While the changes here described take place, 

 the zona pellucida increases in thickness. 



With reference to the yolk Valentin J stated that it was richer in 

 granules the younger the ovum. But BischofT affirms that the reverse of 

 this is the fact. He says, that in almost all animals the number of the 

 granules of the yolk is greater the more mature the ovum, and that the 

 yolk consequently is more opaque in the mature, and more transparent in 

 the immature ova. The matter in which the granules are contained is, 

 according to Bischoff, fluid in the immature ova of all animals. In some 

 it remains so ; but in others, as the human ovum, it subsequently becomes 

 a consistent gelatinous substance. 



* See Bischoff, Barry, Henle, Wagner, Valentin, operibus citatis. 



t Muller's Archiv. 1838, p. 533. f Muller's Archiv. p. 534, 1838. 



Entwick. d. Kaninchen-eies, p. 9; and Entwick. des Hunde-eies, 1845, p. 8. 



