46 THE OVUM AND 



• 



in size. Such a circumstance is, however, very rare, and the 

 weight of evidence before us is much in favour of the second 

 view ; but in order to determine this point, it is necessary to . 

 inquire whether the vesicle exist before the cell. That such 

 is the case is not yet proved, although Baer and Purkinje sup- 

 pose it to be so, and an observation of Wagner's favours the 

 supposition. (Prodromus Physiologias Generationis, p. 9, fig. 

 xviii, a.) He found the posterior extremity of the oviduct of 

 Acheta campestris full of germinal vesicles, which became gra- 

 dually expanded in their progress through the oviduct. The 

 oviduct becomes dilated in its further course ; globules are 

 observed in it, which. Wagner regards as yelk-globules, and 

 between them lie the germ- vesicles; then " each vesicle becomes 

 surrounded by its yelk and chorion, and thus the individual 

 ova become separated." He does not state, however, in what 

 manner the vitelline-membrane is produced. Is it formed as 

 a cell, at first narrowly encompassing the germ-vesicle, and 

 then gradually expanding; or does it at the same time enclose 

 a quantity of the surrounding yelk -globules ? It is difficult 

 to conceive the latter mode of formation ; but if the former be 

 the correct one, the globules surrounding the germ-vesicles in 

 the oviduct cannot be yelk -globules. Fresh researches are 

 therefore necessary, which, if they should be confirmatory of 

 the first view, will also be decisive for considering the germ- 

 vesicle as a cell-nucleus. 1 



With regard to the second point, — namely, as to whether the 

 germ-vesicle be more or less intimately connected with the 

 membrane of the yelk-cell at an early period, or lie free within 

 it, — any evidence afforded by its solution would be comparatively 

 inconclusive. According to Baer and Wagner, the vesicle in 

 the first instance lies in the centre of the velk-cell, and onlv 

 rises to its wall at a later period. Baer quotes the ova of 

 frogs as examples in which it lies for a long time in the centre 

 of the yelk. The germ- vesicle is generally found on the wall 

 of the cell; and in birds, according to Purkinje, it is frequently 

 so intimately connected with it, that it tears in the attempt to 



1 See the Supplement. The observations of Wagner upon the ova of insects 

 which are there quoted, and the recent researches of Barry on those of mammalia 

 and birds, (1. c. p. 308,) prove the germinal vesicle to be first formed, and then the 

 vitelline membrane round it. 



