186 
MR. NEWPORT ON THE IMPREGNATION OF 
to have some reference to changes in the interior of the yelk, possibly to some rapid 
evolution of the so-called central or embryo vesicle in the locality originally occupied 
by the germinal vesicle and spot, which, as we have seen, is nearest to the dark surface 
— it is the partial rotation of the entire yelk. Up to about this period the ova re- 
main undisturbed in the water in a mass as they are expelled, and lie indiscriminately, 
some with the dark and some with the white portion of the yelk uppermost, or hori- 
zontal. But during the time that has passed since the ova have been in contact with 
water, the envelopes have imbibed fluid and expanded until these investments of the 
yelk have acquired a thickness equal to about two-thirds of the diameter of the yelk 
itself. The yelks that have remained to this time with their white surface uppermost 
now change their position spontaneously by a partial rotation of the whole mass of 
each on its axis, within the vitelline membrane, until the dark surface of the whole 
is placed uppermost. Whether this change of position is merely the result of an ex- 
pansion of the vitelline membrane at this period, when the ovum is rapidly ceasing 
to be susceptible of impregnation, as I shall presently show is the case after this lapse 
of time in the water, or whether it be also connected, as we may fairly believe, with 
changes going on in the interior of the yelk, I am not prepared to decide. It is 
important, however, to note that the change takes place at about the time at which I 
have found a great abundance of bright clear rounded vesicles distributed throughout 
the yelk, but chiefly in the place originally occupied by the germinal vesicle. In some 
of these vesicles, which I regard as the progeny of the germinal vesicle, I have seen 
irregular-shaped nuclei that appeared to be formed of a multitude of nucleoli. These 
vesicles convey to me the same idea as those seen by Bischoff in mammalian ova, 
excepting only that in the egg of the Frog they contain compound nuclei. 
At thirty minutes the central patch on the white surface of the egg has almost dis- 
appeared, and the halo around it is still more diffused. At forty-Jive minutes it has 
entirely disappeared in most specimens, and its place is occupied by a broad dark 
area which includes the boundary of the previous halo, and which appears to be 
occasioned by a slight depression in the centre of this surface of the yelk. One hour 
after spawning this depression is somewhat deeper. The white surface has become 
still more defined, and the dark has acquired a more intensely black colour. The 
egg remains in this state without further perceptible change during the succeeding 
second and third hour, excepting only that the depression in the white surface becomes 
a little deeper, but it has almost disappeared at the end of the fourth hour, when seg- 
mentation or cleavage of the yelk is about to take place. But this is not invariably 
the case. When it does remain it is always of an oval form, and the primary cleavage 
of the yelk, as it proceeds on either side from above downwards, meets in its centre 
and invariably passes through it transversely to its long diameter. These are the first 
perceptible changes in ova that are impregnated by the natural union of the sexes, 
and when spawning has not been retarded. But in some broods of eggs that have 
been retained longer than usual in the oviducts, the whole of these changes have 
