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 Physiologiz 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 yelk-cell, and only 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
- ↑ 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, (l. c. p. 308,) prove the germinal vesicle to be first formed, and then the vitelline membrane round it.
