566 GENERATION AND DEVELOPMENT. 



With regard to the parts of the ovum first formed, it appears 

 certain that the formation of the germinal vesicle precedes 

 that of the yelk and zona pellucida, or vitelline membrane. 

 Whether the germinal spot is formed first, and the germinal 

 vesicle afterwards developed around it, cannot be decided in 

 the case of vertebrate animals ; but the observations of Kol- 

 liker and Bagge on the development of the ova of intestinal 

 worms show that in these animals, the first step in the process 

 is the production of round bodies resembling the germinal 

 spots of ova, the germinal vesicles being subsequently devel- 

 oped 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 component parts, 

 consist in alterations of the size and position of these parts 

 with relation to each other, and of the ovum itself with rela- 

 tion to the Graafian vesicle, and in the more complete elabora- 

 tion of the yelk. The earlier the stage of development the 

 larger is the germinal vesicle in relation 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 periphery. 



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 imbedded in a thickened por- 

 tion of the membrana granulosa, it is possible 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 thick- 

 ness. 



According to Bischoff, the number of the granules of the 

 yelk is greater the more mature the ovum, consequently the 

 the yelk is more opaque in the mature, and more transparent 



