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Show 1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 519 problematical structures will be described in this portion of my paper, although they do not appear to belong to the same series as that which I shall now describe. The body (fig. 25) is decidedly smaller than in Protopterus; it corresponds, however, very nearly to the stage illustrated in fig. 9 of Plate LIII., and which is fully described on p. 513, where the slight differences which it presents from other individuals belonging to Stage I. are pointed out. The correspondence is in structure as well as in size. The body is placed at some little distance from the external surface of tbe ovary, but is connected with the germinal epithelium of the surface of the ovary by a slender pedicle of cells; it consists, like the corresponding structure of Protopterus, of a hollow sphere of cells which enclose a central mass ; the peripheral and central cells are also more or less independent. The peripheral layer of cells forms a continuous whole, but a closer examination shows it to be made up of two layers which are occasionally very distinctly separable from each other. The outermost layer appears to have originated from the germinal epithelium; its nuclei are large, rounded, or oval, and closely pressed together ; the cells themselves, which contain these nuclei, have for the most part a stroma-like appearance (see p. 508). Within this layer, which is often several cells thick, is a layer of blood-vessels ensheathed in a mass of tissue of a fibrous retiform character, the fibres (fig. 25, a) for the most part forming a layer running parallel with the circumference of the ovum, with interspersed nuclei; this tissue, from its general appearance and from its intimate connection with the blood-cavities (fig. 25, bl), is probably derived, like the blood-vessels, from ingrowths of stroma. The nuclei of this presumed stroma-layer are oil the whole more elongated in form than those of the outer layer ; the blood-vessels were gorged with blood. The central mass of cells is probably during life in contact with the peripheral layers, but it appeared to be for the greater part at least quite distinct from it, there being no transition between its cells and those of the peripheral layers. The central cells appear to be closely similar in structure to the corresponding cells in Protopterus; the protoplasm of the cell is reticulate with large spaces left between the individual strands which form the network ; the nucleus is of moderate size, round or oval in shape. Here and there (figs. 25, 26, f.e) some of the central cells were disposed in a row, one cell thick, round the periphery, closely applied to the innermost (stroma) layer of the peripheral layers. This is possibly to be compared to the true follicle-layer in Protopterus. Although, as I have already stated, the material at m y disposal in the case of Ceratodus was not well preserved, certain portions of the ovary were in a better condition than others, and, generally speaking, it was quite possible to make out the relations of the different parts of the organ, both the stroma and contained ova, as well as occasionally the germinal epithelium on the outside; the minutiae of structure of the different cells were disguised by the inferior |