| OCR Text |
Show 1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 511 the opinion that it may be derived from the germinal epithelium. Owsiannikow suggests three possibilities 1-either it originates from cells which have made their way out of the blood-vessels (!), or from cells of the subgerminal tissue (stroma?), or, finally, they may be derived from the germinal layer. The latter alternative is adopted by Owsiannikow on certain evidence, which he does not, however, regard as conclusive. The outermost follicular layer of Lepidosiren I have already (p. 509) shown without doubt to be derived from the germinal epithelium ; I shall therefore adopt the name of secondarg follicular epithelium for this cipsule, which indicates that its origin is similar to that of the true follicular epithelium, which may be briefly termed the follicular epithelium. The two layers that have just been described form a hollow sphere enclosing a central cavity, which is partly occupied by a mass of cells. It is very possible that in the fresh condition the central mass of cells occupies the whole of the space available, but this is not the case in m y preparation. A large portion of the central cavity, particularly on the side turned towards the exterior of the ovary, is quite empty, and no structures intervene between the central mass of cells and the follicular layer. O n the opposite side, however, the central mass is in close contact for a considerable area with the follicular cells, this area exactly corresponding with the transitional area between the follicular and external layers. These facts would suggest that the central cells are derived from the proliferation of the follicular cells and ultimately of the extra-follicular cells, as these two latter have been shown to be perfectly continuous, the proliferation taking place in a certain limited area only. In this case the apparent cavity which separates the central cells from tbe follicular on one side will be an indication (exaggerated by the action of the preservative reagent) that there is here no real connection between the central and peripheral layers, though they m a y be in actual contact in the fresh state. A number of the central cells are displayed in figs. 14-20 of Plate LIII.; they are more or less irregular in shape, rounded, and of different sizes; the staining-reagent has hardly affected the cell-protoplasm, but has deeply stained the nucleus. The cell-protoplasm is arranged in a reticulate fashion, and closely resembles that of the follicular cells. Some of the cells contain two or more nuclei, which seems to show that the cells themselves are in a condition of multiplication. The most remarkable fact about the nuclei of the central plug of cells is their great inequality in size : some of the variations are exhibited in those figures ; the variation is all the more remarkable as it does not occur in the follicular layer, the nuclei of whose cells are of quite a uniform size. There is almost every gradation in size between the smallest and largest nuclei, a fact which perhaps indicates that the smaller ones are the result of nuclear division. The largest nuclei rather excel in size those of the follicular epithelium. There is a similar difference of size in the peripheral layer of cells, particularly obvious at those points where the peripheral layer is in 1 Loc. cit. p. 30. |
