This list is sufficient to shew that the disappearance of the germinal vesicle before impregnation is very common, and I am unacquainted with any observations tending to shew that its disappearance is due to impregnation.
In some cases, e.g. Asterocanthion[78], the germinal vesicle vanishes after the spermatozoa have begun to surround the egg; but I do not know that its disappearance in these cases has been shewn to be due to impregnation. To do so it would be necessary to prove that in ripe eggs let loose from the ovary, but not fertilized, the germinal vesicle did not undergo the same changes as in the case of fertilized eggs; and this, as far as I know, has not been done. After the disappearance of the germinal vesicle, and before the first act of division, a fresh nucleus frequently appears [—vide—Auerbach (Ascaris nigrovenosa), Fol (Geryonia), Kupffer (Ascidia canina), Strasburger (Phallusia mamillata), Flemming (Anodon), Götte (Bombinator igneus)], which is generally stated to vanish before the appearance of the first furrow; but in some cases (Kupffer and Götte, and as studied with especial care, Strasburger) it is stated to divide. Upon the second nucleus, or upon its relation to the germinal vesicle, I have no observations; but it appears to me of great importance to determine whether this fresh nucleus arises absolutely de novo, or is formed out of the matter of the germinal vesicle.
The germinal vesicle is situated in a bed of finely divided yolk-particles. These graduate insensibly into the coarser yolk-spherules around them, though the band of passage between the coarse and the finer yolk-particles is rather narrow. The mass of fine yolk-granules may be called the germinal disc. It is not to be looked upon as diverging in any essential particular from the remainder of the yolk, for the difference between the two is one of degree only. It contains in fact a larger bulk of active protoplasm, as compared with yolk-granules, than does the remainder of the ovum. The existence of this agreement in kind has been already strongly insisted on in my preliminary paper; and Schultz (loc. cit.) has arrived at an entirely similar conclusion, from his own independent observations.
One interesting feature about the germinal disc at this period is its size.
My observations upon it have been made with the eggs of the Skate (Raja) alone; but I think that it is not probable that its size in the Skate is greater than in Scyllium or Pristiurus. If its size is the same in all these genera, then the germinal disc of the unimpregnated ovum is very much greater than that portion of the ovum which undergoes segmentation, and which is usually spoken of as the germinal disc in impregnated ova.
I have no further observation on the ripe ovarian ovum; and my next observations concern an ovum in which two furrows have already appeared.
[70] Archiv für Micro. Anat. Vol. XI. 1875.
[71] Quart. Journ. Micro. Science, Oct. 1874. [This edition, No. V.]
[72] Archiv für Micr. Anat. Vol. VIII. p. 1.
[73] Entwicklungsgeschichte der Unke.