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Show 1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 525 changes, though the defeneration of a few out of an immense number (see footnote) is hardly proof of this. If I were in a position to deny the presence of a germinal vesicle, the absence of this essential element in the ovum would be evidence of some force in the same direction. On the other hand, the energy showed by the central cells and the cells of the follicular epithelium would be entirely thrown away in this case ; and it is very difficult to imagine the continuance of such a wasteful process in the ovary-the organ chiefly concerned with the preservation of the race1. A formation of ova in the Vertebrata by a fusion of cells has been stated to exist, but has been subsequently denied. Goette's observations on the formation of the ovum in Bombinator2, referred to in my former paper, have been explained away by Nussbaum 3, who considers that the polynuclear condition described by Goette is the result of the proliferation of the nucleus of a primitive germinal cell, and is not produced by the approximation of the nuclei of a number of distinct cells which subsequently are fused. The observations of v. Siebold4 are of interest in relation to this question. In Apus the ova are formed in spherical acini which contain a number of cells of which one grows at the expense of the rest. This cell finally comes to occupy the whole of the interior of its acinus, the remaining cells dwindling indefinitely ; the nucleus disappears, and yolk-formation sets in. When the cell has been, for the most part, converted into yolk it moves down the duct which connects the acinus with one of the branching tubes of the ovary, and there fuses with one or more cells which have been produced in other acini by a similar course of development. The fused mass becomes surrounded by a membrane, and is the ovum. In my preliminary notice in the ' Zoologischer Anzeiger' I have referred to the observations of v. Siebold, and differ very widely from a description (Arch, fiir mikr. Anat. 1886) of the degenerating ovum of the trout. I may also take this opportunity of referring to an ovum of Ceratodus in a similar condition of degeneration. I erroneously mentioned this ovum (Zool. Anzeig. No. 236) as a nearly fully mature ovum belonging to the multicellular or plasmodial type; it may very likely belong to tins type, but the appearances which it presents are indicative of degeneration, and not of maturation. The follicular epithelium is not separated by any membrane from the contents of the ovum (fig. 2 ) ; its cells in a few cases are loaded with brown pigment, and many of them have migrated into tbe ovum, the contents of which have been largely absorbed, probably by these cells; in consequence of this the ovum is collapsed. I have noticed a similar state of affairs in the ovary of a newt (Molge waltlii), which I purpose to describe on some future occasion. 1 It has been suggested to me, in conversation, by Prof. Lankester that these structures may serve as food for the embryos, being deposited along with the ova, or that they may be reservoirs of nutritive material aiding in the growth of the intra-ovnrian ova. Either of these suggestions appears to me to be extremely plausible. a Arch. f. mikr. Anat, Bd. xviii. 3 Entwickelungsgescbichte der Unke. 4 Beitrage BUT Parthenogene.se der Arthropoden. Leipsic, 1871. |