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 are removed to make room for the supply of the necessary parts to it again by the spermatic nucleus (vide p. [541]). More light on this, as on other points, may probably be thrown by further investigations on parthenogenesis and the presence or absence of a polar cell in eggs which develop parthenogenetically. Curiously enough the two groups in which parthenogenesis most frequently occurs in the ordinary course of development (Arthropoda and Rotifera) are also those in which polar cells, with the possible exception mentioned above, of the parthenogenetic eggs of Lacenularia, are stated to be absent. This curious coincidence, should it be confirmed, may perhaps be explained on the hypothesis, I have just suggested, viz. that a more or less essential part of the nucleus is removed in the formation of the polar cells; so that in cases, e.g. Arthropoda and Rotifera, where polar cells are not formed, and an essential part of the nucleus not therefore removed, parthenogenesis can much more easily occur than when polar cells are formed.
That the part removed in the formation of the polar cells is not absolutely essential, seems at first sight to follow from the fact of parthenogenesis being possible in instances where impregnation is the normal occurrence. The genuineness of all the observations on this head is too long a subject to enter into here[366], but after admitting, as we probably must, that there are genuine cases of 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 less possible.
The remarkable observations of Professor Greeff (19) on the parthenogenetic development of the eggs of Asterias rubens tell, however, very strongly against this explanation. 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. Professor 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.
It is possible that the removal of part of the protoplasm of the egg in the formation of the polar cells may be a secondary process due to an attractive influence of the nucleus on the cell protoplasm, such as is ordinarily observed in cell division.
Impregnation of the Ovum.
A far greater amount of certainty appears to me to have been attained as to the effects of impregnation than as to the changes of the germinal vesicle which precede this, and there appears, moreover, to be a greater uniformity in the series of resulting phenomena. For convenience I propose to reverse the order hitherto adopted and to reserve the history of the literature and my discussion of disputed points till after my general account. Fol's paper on Asterias glacialis, is again my source of information. The part of the germinal vesicle which remains in the egg, after the formation of the second polar cell, becomes converted into a number of small vesicles (Fig. 10), which aggregate themselves into a single clear nucleus which gradually travels toward the centre of the egg and around which as a centre the protoplasm becomes radiately striated (Fig. 11). This nucleus is known as the female pronucleus[367]. In Asterias glacialis the most favourable period for fecundation is about an hour after the formation of the female pronucleus. If at this time the spermatozoa are allowed to come in contact with the egg, their heads soon become enveloped in the investing mucilaginous coat. A prominence, pointing towards the nearest spermatozoon, now arises from the superficial layer of protoplasm of the egg and grows till it comes in contact with the spermatozoon (Figs. 12 and 13), Under normal circumstances the spermatozoon, which meets the prominence, is the only one concerned in the fertilisation, and it makes its way into the egg by passing through the prominence. The tail of the spermatozoa, no longer motile, remains visible for some time after the head has bored its way in, but its place is soon taken by a pale conical body which is, however, probably in part a product of the metamorphosis of the tail itself (Fig. 14). This body vanishes in its turn.
Fig. 12.
