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Show 290 MR. F. E. BEDDARD ON THE [May 4, lopment of Bombinator1; but there are certain facts adduced by Gotte which require to be recorded in Lepidosiren before m y opinion can be expressed without hesitation. Gotte observed a fusion of the nuclei as well as of the protoplasm of the primitive germinal cells ; I have not been able to detect any such fusion in Lepidosiren; on the other hand it is difficult to interpret m y results so as to regard these ova as equivalent in each case to a single cell. As already mentioned, the series of ova between the earliest stage where a mass of follicular cells contained a nest of primitive germinal cells, and the latest stage when the ovum was surrounded by a single layer of follicular cells and separated from them by a delicate membrane, is fairly complete ; there seems to be no doubt that somehow or other the numerous primitive germinal cells of the nest result in the formation of one ovum. There were no signs of the separation of any of these cells to form a number of different ova, as Balfour has described in Scyllium; none of the central cells became larger than the rest, and the follicular epithelium, which was specialized as such in the earliest stage observed, showed no breaks in its continuity at any time ; if the ovum really represents one of the primitive germinal cells, the rest serving as pabulum, it is remarkable to find that this cell shows no difference in size or structure from the rest as development proceeds. The appearance of a fusion of some of the cells, which strongly supports m y view of the case, is illustrated in Plate XXVIII. fig. 7, x; the homogeneous mass depicted in that figure contains a few nuclei, and occasionally a certain amount of the cell-protoplasm still unaltered centrally, but peripherally shading off into the supposed protoplasmic mass. It might perhaps be supposed that this peripheral darkly-staining mass is really due to the coagulation of some fluid substance ; but in that case the appearance of nuclei, without any cell-protoplasm in its interior, would have to be explained, and more particularly the presence of yolk-particles which seem to be formed by its metamorphosis. For the present I am unable to say any more about the development and maturation of these ova; the most important matter that requires further investigation is the mode of origin of the germinal vesicle; but there are obviously other points that remain to be discovered before the whole history of these ova can be cleared up. It might be supposed that the facts described in this section do away with the necessity of any immigration of follicular cells; the yolk has been shown to originate largely in the primitive germinal cells, and many of the free cells remaining in the interior of the ovum during later stages are no doubt derived directly from the primitive germinal cells. In the earlier stages the follicular cells, although forming a distinct layer, are not individually very different from the central cells of the nest; hence their proliferation and migration inwards is not surprising. It has been already mentioned that the yolk commences to be formed very early in the follicular cells as well as in the central cells, so that the former evidently takes a 1 ' Entwickelungsgeschichte der Unke.' |