OCR Text |
Show 510 MR. F. E. BEDDARD ON THE DEVELOPMENT AND [Dec. 7, the follicular cells, but serves at any rate to determine accurately the boundary line between the follicular layer proper and the mass of peripheral cells which ultimately bear a resemblance to the secondary follicle layer. This limiting band of structureless substance is only met with "on that side of the cell-mass towards the outside of the ovary ; elsewhere the cells of the follicular epithelium are perfectly continuous with the cells of the peripheral layer. The nature of the cells of the follicular layer also differs from that of the more peripherally placed cells. The cells themselves are rather larger and irregularly rounded in form ; there is no trace of the connective- tissue-like structure described above, in the case of the extra-follicular cells. The cell-contents are clear and for the most part hardly affected by the staining-agent, which has coloured their nuclei deeply; the part of the cell-protoplasm that is coloured is tinged very faintly and sbows a reticulate arrangement. The nuclei of the follicular cells differ for the most part from those of the extra-follicular layer by being rounded and even in shape, and all closely similar in size ; they are deeply stained, and show a tendency to the same reticulate arrangement of the nuclear substance that has already been mentioned in the extra-follicular cells. The character of this follicular layer is m u c h the same throughout, only differing in places by the more or less crowded condition of the nuclei, indicating a more or less active multiplication of the cells. On that side of the cellular mass which is furthest from the outside of the ovary, the follicular layer comes into closer relations with the extra-follicular epithelial layer, though still recognizable. The character of the cells and of the nuclei which make up the extra-follicular coat of cells alters, and every transitional condition is met with between these cells and the cells of the follicular epithelium. This seems to indicate that the follicular layer is formed as a differentiation of the mass of invaginated germinal cells. The whole body is thus surrounded by three distinct and independent layers-(1) the single layer of large follicular cells ; (2) a vascular layer, to which reference has already been made and which is extremely developed ; (3) an outermost cellular layer, consisting of flattened cells with nuclei elongated in the direction of the circumference of the ovum ; this layer, like the follicular layer, is only one cell thick. For the most part this outer layer has been neglected by writers, or else has been confused with the true follicular layer. Balfour, however, has recognized it in the Elasmobranch ovum x and has proposed to call it the "secondary follicle-layer." Owsiannikow figures this layer in the ovum of the Perch 2; in the explanation of the figure it is called the follicular layer, while the true follicular cells are termed the "granulosa" ; in the text of his paper, however, the term endothelium is constantly used for this layer, which is stated to be made up of several rows of cells in many fishes. Concerning the origin of this layer Balfour expresses with hesitation 1 Quart. Journ. Micr. Sci. vol. xviii. (1878), p. 405, ph xix. fig. 29, fe'. 2 M6m. Acad. Sci. St. P6tersbourg, t. xxxiii. (1885) no. 4, pi. i. fig. 4, a. |