Though there are reasons for doubting the accuracy of all the above details on the ovum of the rabbit, nevertheless, the observations of van Beneden taken as a whole afford strong grounds for concluding that the formation of the polar cells is connected with the disappearance, partial or otherwise, of the germinal vesicle. A very similar account of the apparent disappearance of the germinal vesicle is given by Greeff (19) who states that the apparent disappearance of the germinal spot precedes that of the vesicle.
The observations of Bütschli are of still greater importance in this direction. He has studied with a view to elucidating the fate of the germinal vesicle, the eggs of Nephelis, Lymnæus, Cucullanus, and other Nematodes; and Rotifers. In all of these, with the exception of Rotifers, he finds polar bodies, and in this respect his observations are of value as tending to shew the widespread existence of these structures. Negative results with reference to the presence of the polar bodies have, it may be remarked, only a very secondary value. Bütschli has made the very important discovery that in perfectly ripe eggs of Nephelis, Lymnæus and Cucullanus and allied genera a spindle, similar to that of ordinary nuclei in the act of division, appears close to the surface of the egg. This spindle he regards as the metamorphosed germinal vesicle, and has demonstrated that it takes part in the formation of the polar cells. He states that the whole spindle is ejected from the egg, and that after swelling up and forming a somewhat spherical mass it divides into three parts.
In the Nematodes generally, Bütschli has been unable to find the spindle modification of the germinal vesicle, but he states that the germinal vesicle undergoes degeneration, its outline becoming indistinct and the germinal spot vanishing. The position of the germinal vesicle continues to be marked by a clear space which gradually approaches the surface of the egg. When it is in contact with the surface a small spherical body, the remnant of the germinal vesicle, comes into view, and eventually becomes ejected. The clear space subsequently disappears. This description of Bütschli resembles in some respects that given by van Beneden of the changes in the rabbit's ovum, and not impossibly refers to a nearly identical series of phenomena. The discovery by Bütschli of the spindle and its relation to the polar body has been of very great value.
The publications of van Beneden, and more especially those of Bütschli, taken by themselves lead to the conclusion that the whole germinal vesicle is either ejected or absorbed. Nearly simultaneously with their publications there appeared, however, a paper by Oscar Hertwig (11) on the eggs of one of the common sea urchins (Toxopneustes lividus), in which he attempted to shew that part of the germinal vesicle, at any rate, was concerned in the formation of the first segmentation nucleus. He believed (though he has himself now recognised that he was in error on the point) that no polar cell was formed in Toxopneustes, and that the whole germinal vesicle was absorbed, with the exception of the germinal spot which remained in the egg as the female pronucleus.
The following is the summary which he gives of his results, pp. 357-8.
“At the time when the egg is mature the germinal vesicle undergoes a retrogressive metamorphosis and becomes carried towards the surface of the egg by the contraction of the protoplasm. Its membrane becomes dissolved and its contents disintegrated and finally absorbed by the yolk. The germinal spot appears, however, to remain unaltered and to continue in the yolk and to become the permanent nucleus of the ripe ovum capable of impregnation.”
After the publication of Bütschli's monograph, O. Hertwig (12) continued his researches on the ova of Leeches (Hæmopis and Nephelis), and not only added very largely to our knowledge of the history of the germinal vesicle, but was able to make a very important rectification in Bütschli's conclusions. The following is a summary of his results:—The germinal vesicle, as in other cases, undergoes a form of degeneration, though retaining its central position; and the germinal spot breaks up into fragments. The stages in which this occurs are followed by one when, on a superficial examination, the ovum appears to be absolutely without a nucleus; but there can be demonstrated by means of reagents in the position previously occupied by the germinal vesicle a spindle nucleus with the usual suns at its poles, which Hertwig believes to be a product of the fragments of the germinal spot. This spindle travels towards the periphery of the ovum and then forms the spindle observed by Bütschli. At the point where one of the apices of the spindle lies close to the surface a small protuberance arises which is destined to form the first polar cell. As the protuberance becomes more prominent one half of the spindle passes into it. The spindle then divides in the normal manner for nuclei, one half remaining in the protuberance, the other in the ovum, and finally the protuberance becomes a rounded body united to the egg by a narrow stalk. It is clear that if, as there is every reason to think, the above description is correct, the polar cell is formed by a simple process of cell-division and not, as Bütschli believed, by the forcible ejection of the spindle.
The portion of the spindle in the polar cell becomes a mass of granules, and that in the ovum becomes converted without the occurrence of the usual nuclear stage into a fresh spindle. A second polar cell is formed in the same manner as the first one, and the first one subsequently divides into two. The portion of the spindle which remains in the egg after the formation of the second polar cell reconstitutes itself into a nucleus—the female pronucleus—and travelling towards the centre of the egg undergoes a fate which will be spoken of in the second part of this paper.
The most obscure part of Hertwig's work is that which concerns the formation of the spindle on the atrophy of the germinal vesicle, and his latest paper, though it gives further details on this head, does not appear to me to clear up the mystery. Though Hertwig demonstrates clearly enough that this spindle is a product of the metamorphoses of the germinal vesicle, he does not appear to prove the thesis which he maintains, that it is the metamorphosed germinal spot.
Fol, to whom we are indebted in his paper on the development of Geryonia (7) for the best of the earlier descriptions of the phenomena which attend the maturation of the egg, and later for valuable contributions somewhat similar to those of Bütschli with reference to the development of the Pteropod egg (8), has recently given us a very interesting account of what takes place in the ripe egg of Asterias glacialis (9). In reference to the formation of the polar cells, his results accord closely with those of Hertwig, but he differs considerably from this author with reference to the preceding changes in the germinal vesicle. He believes that the germinal spot atrophies more or less completely, but that in any case its constituents remain behind in the egg, though he will not definitely assert that it takes no share in the formation of the spindle at the expense of which both the polar cells and the female pronucleus are formed. The spindle with its terminal suns arises, according to him, from the contents of the germinal vesicle, loses its spindle character, travels to the surface, and reacquiring a spindle character is concerned in the formation of the polar cells in the way described by Hertwig.