The works of Francis Maitland Balfour, Volume 2 (of 4)

by Francis M. Balfour

Whatever may be the eventual result of more extended investigation, it is clear that the formation of polar cells according to the type described above is a very constant occurrence. Its importance is 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, etc., but, according to our present knowledge, far more usually after the ovum has been laid. In some instances the budding-off of the polar cells precedes, and in other instances follows impregnation; but there is no evidence to shew that in the latter cases the process is influenced by the contact with the male element. In Asterias, as has been shewn by O. Hertwig and Fol, 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 bodies 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.

I would suggest that in the formation of the polar cells part of the constituents of the germinal vesicle, which are requisite for its functions as a complete and independent nucleus, is removed, to make room for the supply of the necessary parts to it again by the spermatic nucleus.

My view amounts to the following, viz. that after the formation of the polar cells the remainder of the germinal vesicle within the ovum (the female pronucleus) is incapable of further development without the addition of the nuclear part of the male element (spermatozoon), and that if polar cells were not formed parthenogenesis might normally occur. A strong support for this hypothesis would be afforded were it to be definitely established that a polar body is not formed in the Arthropoda and Rotifera; since the normal occurrence of parthenogenesis is confined to these two groups. It is certainly a remarkable coincidence that they are the only two groups in which polar bodies have not so far been satisfactorily observed.

It is perhaps possible that the part removed in the formation of the polar cells is not absolutely essential; and this seems at first sight to follow from the fact of parthenogenesis being possible in instances where impregnation is the normal occurrence. The genuineness of the observations on this head is too long a subject to enter into here[32], but after admitting, as we probably must, that there are genuine cases of such parthenogenesis, it cannot be taken for granted without more extended observation that the occurrence of development in these rare instances may not be due to the polar cells not having been formed as usual, and that when the polar cells are formed the development without impregnation is impossible.

Selenka found in the case of Purpura lapillus that no polar body was formed in the eggs which did not develop, but in the case of Neritina, Bütschli has found that this does not hold good.

The remarkable observations of Greeff (No. [88]) on the parthenogenetic development of the eggs of Asterias rubens tell, however, very strongly against the above hypothesis. Greeff has found that under normal circumstances the eggs of this species of starfish will develop without impregnation in simple sea-water. The development is quite regular and normal, though much slower than in the case of impregnated eggs. It is not definitely stated that polar cells are formed, but there can be no doubt that this is implied. Greeff’s account is so precise and circumstantial that it is not easy to believe that any error can have crept in; but neither Hertwig nor Fol have been able to repeat his experiments, and we may be permitted to wait for further confirmation before absolutely accepting them.

To the suggestion already made with reference to the function of the polar cells, I will venture to add the further one, that the function of forming polar cells has been acquired by the ovum for the express purpose of preventing parthenogenesis.