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-five 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 denned, 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 hi its centre 

 and in variably 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 



