Memoirs: Notes on the Maturation of the Ovum of Alcyonium digitatum

Although the foregoing accouut is obviously incomplete, yet it is possible to draw some conclusions. I believe we have shown that: (a) The egg of Aleyonium produces no polar bodies in the ordinary sense of the word. (b) That the division of the female pronucleus before the entrance of the spermatozoon is irregular and amitotic. (c) That no chromatiu leaves the egg in the stage of ovocyte I, to use Boveri's nomenclature. (d) That the female pronucleus completely disappears. (s) That there are no bodies that can be termed chromosomes throughout the whole process. (f) That the first segmentation nucleus is formed in a way (unknown) that must in any case be unlike anything hitherto described. Furthermore I believe, though I cannot state so positively, that: (g) A process takes place that may roughly be compared to the formation of polar bodies, but they disintegrate and do not leave the cell. So far as I have ascertained nothing in the shape of extrusion takes place. It is, however, curious to note that the nearest account that I have been able to find of like behaviour in an egg nucleus is that of Sfcoeckel.1 This author found in a human ovary certain of the ova containing large nuclei, the membrane of which, as a rule, was well marked, but "oft geht diese scharfe Abgrenzung auch verloren. Die Konturen werden unregelmässeg Zackig, verschwommen."...Stoeckel believed that these changes in form were the beginning of amitotic nuclear divisions, giving rise to binucleated ova, of which he found several. I know of no other assumption of the direct division of an egg nucleus before fertilisation. (h) The first segmentation nucleus is derived from the male pronucleus, though it is quite possible that chromatin equal in amount to that of the male may also be derived from the female pronucleus, though all trace of the latter has been lost. If the foregoing statements be only partially true, it is obvious that a great gulf is fixed between the maturation processes in the egg of Alcyonium and all hitherto described cases. For this reason I am very loth to do more than state the bare conclusions to which I have come. To the best of my belief, no author has described amitotic nuclear divisions in the formation of polar bodies. We seem to have to deal with perfectly abnormal conditions. Nevertheless we are forced to admit that the maintenance of the individual parental chromosomes in a fertilised egg-cell is not universal. On the contrary, all trace of the chromatin of the original egg nucleus is lost. Furthermore, there are several instances among the protozoa of the breaking up and reorganisation of the nucleus. This occurs in many Ciliates. For instance : In ‘Oxytricha and Lacrymaria’ Gruber has shown that the meganucleus breaks up into minute fragments which become scattered through the protoplasm, but eventually reunite into a single body. So much attractive speculation has been based on the ordinarily observed facts of maturation and fertilisation that we feel almost bound to assume that these processes are the same iu all metazoa. But it is obvious that in certain cœlenterates we have facts before us that cannot be brought into line with what we feel we have a right to expect. At present the affair is a mystery. Pending farther investigation it were unwise to speculate on the possible meaning of these phenomena, however much one may be tempted to do so. In case that what I have observed, and still more failed to observe, may induce some other zoologist to follow this cytological byeway, I can only hope that he will find this paper a path over which it may not be necessary to retrace his steps. There would seem to be four points to which attention should be specially directed: (a) The nuclear history of the germ-cells from their earliest "Anlagen." (b) The mode of formation of the polar bodies. (c) The actual penetration of the spermatozoon. (d) The way in which the first segmentation nucleus is built up. Lastly, I have but to express my sincere thanks to Professor Hickson for his kindness in allowing me the use of his material, preparations, and notes, and for the many fruitful suggestions that I have received from him.