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 Physiologiz Generationis, p. 9, fig. 
xvii, 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.' 
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 im 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, (1. c. p. 308,) prove the germinal vesicle to be first formed, and then the 
vitelline membrane round it. 
