722 GENERATION AND DEVELOPMENT. 



germinal vesicles being subsequently developed around 

 these in the form of transparent membranous cells. 



The more important changes that take place in the 

 ovum next to the formation of these its essential com- 

 ponent parts, consist in alterations of the size and position 

 of these parts with relation to each other, and of the ovum 

 itself with relation to the Graafian vesicle, and in the more 

 complete elaboration of the yelk. The earlier the stage of 

 development the larger is the germinal vesicle in rela- 

 tion to the whole ovum, and of the ovum in relation to 

 the Graafian vesicle. For, as the ovum becomes mature, 

 although all these parts increase in size, the Graafian 

 vesicle 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 

 vesicle, but subsequently is removed to its periphery. 

 The germinal vesicle, too, which in young ova is in the 

 centre of the yelk, is in mature ova found at the peri- 

 phery. 



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

 the periphery of the Graafian vesicle, is possibly connected 

 with the formation of the membrana granulosa which lines 

 the vesicle. For, according to Valentin, at a very early 

 period, the contents of the vesicle between its wall and the 

 ovum are almost wholly formed of granules ; but in the 

 process of growth a clear fluid collects in the centre of the 

 vesicle, 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 em- 

 bedded in a thickened portion of the membrana granulosa, 

 it is possible that when the elementary parts of this mem- 

 brane 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 iacreases in thickness. 



