| OCR Text |
Show 1886.] OVUM OF LEPIDOSIREN. 279 IV. The Follicular Epithelium and the Formation of Yolk. In the youngest ova (Plate XXIX. fig. 8) the protoplasm is dense and solid, staining deeply with borax carmine; the uniform appearance of the egg-contents indicates that no formation of yolk has at present commenced. In the next stage, where the zona radiata is well developed (Plate X X V I I I . fig. 1), the protoplasm of the ovum is less dense and has acquired here and there a reticulate arrangement, which is well shown in the figure referred to. In ova slightly more mature (Plate XVIII. fig. 2) the formation of the yolk is in active progress, though for the present confined to limited areas of the egg-protoplasm. None of m y sections display any ova which show the first beginning of yolk-formation. In Plate XXVIII. fig. 2, it will be noted that the yolk appears in patches usually spherical in shape and larger or smaller ; the yolk has the form of minute spherules and aggregations of spherules, which are easily distinguishable from the surrounding protoplasm, which is also granular, by their more coarsely granular appearance. They have also been stained much more darkly by the borax carmine. The yolk-spherules invariably make their appearance in the interspaces between the reticulations of the egg-protoplasm. There is thus no doubt that the yolk is actually formed in the interior of the ovum at the expense of the egg-protoplasm ; the reticulation of the egg-protoplasm, invisible in earlier stages, but completed before the appearance of yolk, is probably a preparation for the formation of the latter, which is received when formed into the interspaces between the protoplasmic strands. Ova of the next stage (Plate XXVIII. fig. 4 ) show the yolk-spherules well developed and filling up the entire ovum; the spherules themselves are small and of varying size ; occasionally numbers of yolk-spherules had run together to form irregular shaped masses ; these yolk-masses, owing to their size and impenetrability, were invariably left unstained. The yolk at this period entirely fills the ovum, and is spread throughout it in a perfectly uniform fashion ; there was no indication of any peripheral layer free, or nearly free, from yolk. The ovum displayed in Plate XXVIII. fig. 4 is remarkable for the fact that the yolk is distinctly differentiated into two layers-a thin peripheral layer, and a central mass ; the boundary between the two was perfectly distinct, there being an absolute break, a narrow line perfectly free from yolk-spherules ; the outer layer was also rendered more conspicuous by the lighter staining of the reagent. I have noticed several ova among m y sections which displayed this curious differentiation. Throughout its whole development the ovum is surrounded by a single layer of follicular epithelium-cells; these cells are flattened as in the Amphibia, and have a large deeply staining nucleus. In most cases a membrana propria folliculi could be detected outside these cells. |
