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Show 516 MR. F. E. BEDDARD ON THE DEVELOPMENT AND [Dec. 7, by their smaller size, by their very deeply stained protoplasm, and by the fact that they nearly always contain a large number (3-5) of small nuclei close together. I have not yet succeeded in detecting these cells in the follicular layers, but I a m nevertheless inclined to think that they are migratory leucocytes. I a m at a loss, however, to account for the almost universal division of their nuclei into four or five. The presence of leucocytes in almost all the tissues and glands of the body is so well known that I need not give any detailed references ; the presence of these cells is not, however, to be confounded with the migration of follicular cells. If, however, the identification of the follicular epithelium with a layer of immigrated leucocytes be right, there can be no distinction between the two processes. The observations recorded in this paper, however, plainly show that in Protopterus at least there can be no possible confusion between follicular cells and lymphoid corpuscles, which is contradicted by so many other developmental facts. Unless it can be shown that lymphoid cells m a y arise from the direct metamorphosis of germinal epithelial cells it is quite absurd, in the present case at any rate, to allow any homology between follicular cells and immigrated white lymph-corpuscles. Stage III.-In this stage (figured diagrammatically in fig. 3, plate xxviiii. of m y former paper) the follicular epithelium is undivided from the ovum by any trace of membrane ; the cells of which it is composed have dwindled down to a single layer ; their diameter bears a very small proportion to that of the enclosed mass of yolk, which has enormously increased in size. The cells of the follicular epithelium are still filled with yolk-spherules presenting no differences from the yolk-spherules which make up the substance of the contained mass. Their nuclei are conspicuous and round in shape. The follicular cells appear to continue to take a share in the nutrition of the body from the fact that they are large and well developed, and that the interstices of the protoplasmic network are largely filled with yolk-spherules : occasionally (e. y. figs. 7, 8, a) the nuclei of the follicular cells showed signs of degeneration ; this is probably preliminary to the evacuation of the cell-contents into the interior. Here and there the follicular cells appeared to be proliferating, the budded-off cells moving into the interior ; two such instances are shown in figs. 7, 8. It is of course a difficult matter to decide how far the appearances shown in the two figures cited are due to the proliferation and migration inwards of the follicular cells; they might be explained, by reference to earlier stages, as central cells which have still remained in contact with the follicular layer, only that they occur on all sides, and it has already (p. 511) been stated that the central cells are only in contact with the peripheral for a limited area. On the other hand, a careful comparison of the example from which fig. 8 is taken with another in pretty much the same stage of development, only younger, as evinced by its smaller size, reveals the important fact that the larger contains, in any given section, a larger number of cells in its interior than the smaller. The larger was rather more |