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Show 1S86.] OVUM OF LEPIDOSIREN. 289 the peripheral cells, from the germinal epithelium, but that they are first invaginated and subsequently surrounded by the peripheral layer. In a later stage the formation of yolk has commenced, and the cells of the central mass are in places separated from each other by aggregations of yolk-particles, though for the most part the cells remain closely adherent; these latter, however, contain yolk-particles in their interior, and the follicular cells, which still form two or three layers, are also filled with yolk. The later stages have been already referred to (p. 283). It is clear, therefore, that in Lepidosiren there are two kinds of ova; those which arise in the way just described may possibly be confined to the postembryonic period. The mass of central cells with the surrounding follicular layers is clearly comparable to the " Ureiernester," described by Balfour and others, in many Vertebrates, e.g. Elasmobranchs. But although there is this general similarity between the ovary of Lepidosiren and that of Scyllium, there is evidently a very great difference in detail. In the Elasmobranch-ovary the nest of primitive germinal cells is imbedded in a mass which consists of the general undifferentiated cells of the germinal epithelium ; there is no definite follicular layer at this period. The protoplasm of the primitive germinal cells fuse together, and the nuclei multiply ; some of the nuclei degenerate, while others undergo further development, and are eventually separated off from the rest, together with a certain amount of protoplasm, to form ova; the degenerating nuclei are absorbed and aid in the nutrition of the ova. When the ovum is formed, some of the undifferentiated germinal cells range themselves round it and form the follicular layer. A nest gives rise to a variable number of ova. The yolk is formed much later. In Lepidosiren the nest is imbedded in a mass of cells which are definitely marked off from the surrounding cells of the ovary, and can be recognized as the future follicular epithelium ; the protoplasm of at least some of the primitive germinal cells fuse together, and the nuclei appear also to multiply, if not the cells themselves in many cases ; some of the nuclei degenerate (show a paler colour under the influence of borax carmine) and become irregular in shape ; the formation of yolk commences extraordinarily early (as compared with the Elasmobranch) in the mass resulting from the fusion of the peripheral cells, in the remaining cells, and in the follicular layers ; the nest gives rise to but a single ovum. It seems to me impossible to deny that the whole structure (Plate XXVIII. fig. 7), which I have compared to the "nest" of the Elasmobranch ovary, eventually becomes a single ovum ; the question that must first be answered is, does the ovum in this case represent a single cell or is it produced by the fusion of a number of cells ? The only answer to this question that the facts at m y disposal enable me to give is that these ova are formed by a coalescence of a number of cells out of the nest, the remainder serving as pabulum. This opinion is so far confirmatory of Gotte's observations on the deve- |