344 
MR. JONES ON THE IMPREGNATED MAMMIFEROUS OVUM. 
is to be observed. The black blastoderma surrounds the whole yelk, with the excep- 
tion of a small spot* on the opposite side to that where the primitive streak appears. 
In the furthest advanced ova contained in the ovary of the Newt, the blastoderma 
was formed, and I think I perceived the place where the germinal vesicle had been. 
As to the ova of the inarnmifera, I have found many in which there was no germinal 
vesicle, and which certainly had not been impregnated. It is to be remarked that in 
such ova the vitelline grains were for the most part coherent and formed the vesi- 
cular blastoderma. 
It being determined that the disappearance of the germinal vesicle is prior to im- 
pregnation and not dependent on it, the next question which arises is “ how does the 
germinal vesicle disappear ?” My observations on the ova of the water Newt are the 
only ones I have which bear upon this question. From what I have observed in 
them I think the mode of disappearance is the following : 
The vesicle, at first imbedded in the substance of the yelk, approaches more and 
more the surface of it, until it comes to lie immediately underneath the vitellary 
membrane, in the manner represented in fig. 12. The coat of the vesicle having 
now become very soft and weak gives way, and the contained fluid is effused on the 
surrounding surface of the yelk. The coat of the vesicle being of extreme tenuity 
cannot be seen after it has given way. The small depression in which the vesicle 
was situate now forms the cicatricula, fig. 13. 
I think that the fluid contained in the germinal vesicle being effused gives a de- 
gree of consistence to the matter composing the surface of the yelk, and thus pro- 
motes the formation of the blastoderma. 
If then the germinal vesicle is not dependent on impregnation, it may be asked, 
what is the first change which takes place in the ova in consequence of impregnation ? 
Of all ova the ova of the Frog are those in which such change can be most directly 
observed. In them the breaking up of the surface of the yelk into crystalline forms, 
described by Prevost, and Dumas, is the first change I have seen. 
March 1 7 th, 1835. I examined to-day the spawn taken from a Frog yesterday, 
part of which was impregnated and part not ; that which was impregnated presented 
the appearance delineated in fig. 11. The unimpregnated ova presented no change 
The surface of the yelk becomes every day still more broken up, the crystalline 
forms becoming smaller and smaller, until the surface of the black blastoderma ap- 
pears under a magnifying glass like shagreen. The blastoderma, consisting of an 
aggregation of clear globules, different from those of the rest of the yelk, is now fully 
* This small spot of the ova of the frog which is white, (from the exposure of the white yelk,) always tutus 
to the most depending side. The germinal point is thus always uppermost. I turned a mass of spawn upside 
down ; the white spot was exhibited by all, but in a short time the white spot had turned downwards and the 
germinal surface again became uppermost. In this case, does the vitellus alone revolve, or does the vitellus 
and its membrane turn round together in the gelatinous substance surrounding the ovum ? It appeared to me 
that the latter was the way in which the revolution took place. 
