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Show 1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 515 greatly diminished; it forms only a single layer of cells, and the pedicle which connects the body with it is almost entirely composed of stroma-ingrowths, which form for the most part a very lax network of fibres and cells, though denser in the middle, and surrounding a slender cord of germinal cells, the sole remnant of the former epithelial pedicle. In a somewhat more advanced stage there is an identical structure with that just described as regards the specialization of the peripheral layers and the mass of central cells ; the formation of yolk has, however, gone on much faster, and the follicular cells, as well as most of the cells of the central mass, are crammed with variously sized yolk-spherules. Here and there, especially in the periphery of the central cells, are irregularly shaped masses of yolk-spherules (fig. 24, a), among which are occasionally discernible nuclei like those of the surrounding cells. A comparison of Plate LIV. fig. 24 with Plate LITI. fig. 11 will show how very similar in size and extent these irregular patches of yolk are to tbe patches of amorphous deeply staining protoplasm in the earlier stage; and 1 cannot but think that they are these same patches of protoplasm produced by the fusion of some of the central cells which have commenced to form yolk-particles on their own account. Other preparations, again, seem to indicate that the fusion of certain of the central cells either does not take place at all, or takes place after the formation of the yolk. I have a nearly complete series of sections through an ovum in which the yolk has already commenced to be formed; the cells of the follicular epithelium are full of yolk-particles, as are also the central cells. The latter form a compact mass of cells containing abundant yolk-particles, but without any definite patches of yolk lying between the cells, such as could be compared to the patches of protoplasm formed by a fusion of cells. In many sections, however, of this series it happened that the central mass of cells had dropped out, so that I cannot make any positive statements about the point of discussion raised. This particular instance showed very plainly indeed the connection of the mass of central cells with the follicular epithelium ; the area of connection was very limited, as has been already mentioned in other cases. The central cells are united with the follicular epithelium by a bridge of tissue which has every appearance of being an outgrowth of the latter; the nuclei are round and pressed close together as if in a condition of active multiplication ; they pass without any break into the follicular epithelium, from the cells of which they cannot, indeed, be distinguished, and gradually on the other side into the mass of central cells. The cells of the latter have the ordinary characters that have been already described. I noticed a very large number of peculiar cells, several of which are illustrated in fig. 4 ; similar cells are not absent in other cases, but I never have seen them so numerous as in the present case. These cells are easily to be distinguished from the germinal cells among which they lie 34* |