Considering how few ova have been adequately investigated with reference to the behaviour of the germinal vesicle any general conclusions which may at present be formed are to be regarded as provisional, and I trust that this will be borne in mind by the reader in perusing the following paragraphs.
There is abundant evidence that at the time of maturation of the egg the germinal vesicle undergoes peculiar changes, which are, in part at least, of a retrogressive character. These changes may begin considerably before the egg has reached the period of maturity, or may not take place till after it has been laid. They consist in appearance of irregularity and obscurity in the outline of the germinal vesicle, the absorption of its membrane, the partial absorption of its contents in the yolk, and the breaking up and disappearance of the germinal spot. The exact fate of the single germinal spot, or the numerous spots where they are present, is still obscure; and the observations of Oellacher on the trout, and to a certain extent my own on the skate, tend to shew that the membrane of the germinal vesicle may in some cases be ejected from the egg, but this conclusion cannot be accepted without further confirmation.
The retrogressive metamorphosis of the germinal vesicle is followed in a large number of instances by the conversion of what remains into a striated spindle similar in character to a nucleus previous to division. This spindle travels to the surface and undergoes division to form the polar cell or cells in the manner above described. The part which remains in the egg forms eventually the female pronucleus.
The germinal vesicle has up to the present time only been observed to undergo the above series of changes in a certain number of instances, which, however, include examples from several divisions of the Cœlenterata, the Echinodermata, and the Mollusca, and also some of the Vermes (Nematodes, Hirudinea, Sagitta). It is very possible, not to say probable, that it is universal in the animal kingdom, but the present state of our knowledge does not justify us in saying so. It may be that in the case of the rabbit, and many Nematodes as described by van Beneden and by Bütschli, we have instances of a different mode of formation of the polar cells.
The case of Amphibians, as described by Bambeke (2) and Hertwig (12) cannot so far be brought into conformity with our type, though observations are so difficult to make with such opaque eggs that not much reliance can be placed upon the existing statements. In both of these types of possible exceptions it is fairly clear that, whatever may be the case with reference to the formation of the polar cells, part of the germinal vesicle remains behind as the female pronucleus.
There are a large number of types, including the whole of the Rotifera[365] and Arthropoda, with a few doubtful exceptions, in which the polar cells cannot as yet be said to have been satisfactorily observed.
Whatever may be the eventual result of more extended investigation, it is clear that the formation of polar cells according to our type is a very constant occurrence. Its importance is also very greatly increased by the discovery by Strasburger of the existence of an analogous process amongst plants. Two questions about it obviously present themselves for solution: (1) What are the conditions of its occurrence with reference to impregnation? (2) What meaning has it in the development of the ovum or the embryo?
The answer to the first of these questions is not difficult to find. The formation of the polar bodies is independent of impregnation, and is the final act of the normal growth of the ovum. In a few types the polar cells are formed while the ovum is still in the ovary, as, for instance, in some species of Echini, Hydra, &c., but, according to our present knowledge, far more usually after the ovum has been laid. In some of the instances the budding off of the polar cells precedes, and in others follows impregnation; but there is no evidence to shew that in the later cases the process is influenced by the contact with the male element. In Asterias, as has been shewn by O. Hertwig, the formation of the polar cells may indifferently either precede or follow impregnation—a fact which affords a clear demonstration of the independence of the two occurrences.
To the second of the two questions it does not unfortunately seem possible at present to give an answer which can be regarded as satisfactory.
The retrogressive changes in the membrane of the germinal vesicle which usher in the formation of the polar bodies may very probably be viewed as a prelude to a renewed activity of the contents of the vesicle; and are perhaps rendered the more necessary from the thickness of the membrane which results from a protracted period of passive growth. This suggestion does not, however, help us to explain the formation of polar cells by a process identical with cell division. The ejection of part of the germinal vesicle in the formation of the polar cells may probably be paralleled by the ejection of part or the whole of the original nucleus which, if we may trust the beautiful researches of Bütschli, takes place during conjugation in Infusoria as a preliminary to the formation of a fresh nucleus. This comparison is due to Bütschli, and according to it the formation of the polar bodies would have to be regarded as assisting, in some as yet unknown way, the process of regeneration of the germinal vesicle. Views analogous to this are held by Strasburger and Hertwig, who regard the formation of the polar bodies in the light of a process of excretion or removal of useless material. Such hypotheses do not unfortunately carry us very far.