GERMINAL MEMBRANE. 43 
membrane, the double outline of which he recognised, (plate 
II, fig. 1, d.) It is thus highly probable that the yelk of the 
mammalian ovum is a cell. Even if, as Wagner intimates, the 
vitellime membrane in other animals should sometimes be 
formed only secondarily within the chorion, it would not 
materially interfere with our purpose, since in that case the 
chorion would be the cell-membrane. ‘The ovum universally 
possesses an external closed membrane (whether it be chorion 
or vitelline membrane), which is structureless, and not gene- 
rated from other elementary structures, and therefore is the 
ovum always a cell. The yelk-cell encloses the vitelline sub- 
stance as its cell-contents, and upon its internal surface lies 
the germinal vesicle, or vesicle of Purkinje, (fig. 1, /) 
This, as is known, is a very transparent thin-walled vesicle, 
containing a pellucid fluid, according to Wagner coagulable by 
spirits of wine. It encloses almost universally (Wagner cites 
but very few exceptions) upon the internal surface of its wall, 
a corpuscle, called by the discoverer, R. Wagner, germinal spot, 
or germinal disc, (fig. 1, g.) Im mammalia it is generally flat. 
In many instances several of these spots are present, their 
number, however, is said by Wagner to bear proportion to the 
age of the ovum, they beg fewer and much more firmly 
attached to the wall of the germinal vesicle in young ova, I have 
frequently observed in osseous fishes (where they are often pre- 
sent in such numbers as to prevent the fluid in the vesicle from 
being seen) that when one of these corpuscles, after the bursting 
of the germ-vesicle, passed through a narrow space, it first 
became considerably elongated, and then drawn out in the 
centre to a thin thread, which soon broke. The two ends 
afterwards retracted, and thus two round globules were pro- 
duced from one corpuscle, in a similar manner to what we may 
observe in the drops of fat upon soup. They appear, therefore, 
to be composed of a tenacious substance which is not miscible 
with water. Purkinje states that the germinal vesicle in birds 
is firmly fixed to the vitellme membrane, but Baer and 
Wagner describe it as lying in the centre of the yelk at first, 
and rising to the surface at a subsequent period. 
The decision of the question, as to the precise signification 
of the germinal vesicle, now becomes of great importance. Is 
it a young cell generated within the yelk-cell, or is it the 
