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Show 1886.] OVUM OF LEPIDOSIREN. 281 The structure of these ova and the surrounding follicle is extremely remarkable, and quite unlike any other ova that I have seen in transverse sections of the ovary of Protopterus. But the peculiarities of these ova do not end here. Scattered throughout the substance of the yolk are an immense number of cells more or less spherical in form (Plate XXVIII. fig. 3, c). These cells were limited in most cases by a very distinct external membrane ; their contents are a mass of rounded bodies which are quite indistinguishable from the yolk-spherules of the ovum, and strands of granular protoplasmic substance ; furthermore there is a distinct and large nucleus deeply stained by borax carmine, and within this a number of nucleoli and granules. These cells appeared to be in a condition of active multiplication, the various stages of which are indicated in Plate X X I X . figs. 9-20. In fig. 10 the nucleus is constricted in the middle; in fig. 11 the nucleus has divided; in fig. 12 the two cells have become distinct but lie close together ; in fig. 18 cell-division has evidently gone on very rapidly, as there is a nest of four cells pressed closely together and occupying a common cavity in the yolk. In some cells the amount of yolk-granules present was considerably less than in others. Occasionally (figs. 19, 20) the yolk-particles had run together to form a single large droplet. In a few cases I observed (fig. 15) the nucleus to be thrust altogether without the cells ; and here and there within the substance of the ovum were apparently nuclei without any cells. I a m not inclined, however, to lay too much stress upon the last mentioned fact, because it would be rather difficult in any case to detect the boundaries of one of these cells if it were packed quite full of yolk-spherules. In other cases (fig. 13) the nucleus had become paler in colour (less acted upon by the staining reagent) as well as more homogeneous. Nuclei of this kind might easily be mistaken for larger yolk-spherules ; indeed m y only reasons for believing them to be altered nuclei are their similiarity in size and shape to the more normal nuclei, their occurrence in cells that have become emptied of their contents, and the fact that they are rather more deeply stained than the surrounding yolk. Concerning the nature of these bodies there appears to m e to be three possibilities :-either (1) they are similar to the white yolk-spherules of the Fowl's egg; or (2) they are follicular cells which have migrated into the interior of the ovum ; or (3) they are cells which have been formed anew within the substance of the ovum. The first alternative, that the structures in question correspond to the white yolk-spheres of birds, seems at first sight to be a likely explanation of their occurrence, especially since they are only to be seen in comparatively young ova. The white yolk-spheres of birds 1 are rounded bodies containing one or more highly refractive nucleuslike bodies in their interior; but these apparent nuclei are really 1 Foster and Balfour's 'Elements of Embryology,! p. 16. PROC. ZOOL. Soc-1886, No. XIX. 19 |