OCR Text |
Show 1886.] OVUxVl OF LEPIDOSIREN. 291 large share in the building up of the ovum ; in later stages, but before the follicular cells have dwindled down to a single layer, there is a considerable proliferation and migration inwards of these cells (Plate XXVIII. figs. 5, 6) ; finally, when the ovum is surrounded by only a single layer of follicular cells, there is still here and there a proliferation of these cells. VI. Resume. The observations recorded in the present paper may be briefly summed up as follows. The ovum is in the earliest stages observed composed of a mass of deeply staining granular protoplasm ; it contains a germinal vesicle limited by a distinct membrane, inside of which is a peripheral layer of germinal spots; the ovum is separated from the surrounding follicular epithelium by a thin structureless membrane. The follicular epithelium is composed of a single layer of cells, each with a large deeply staining nucleus, which retains the same character throughout the whole development of the ovum. In the second stage the protoplasm of the ovum is arranged in a reticulate fashion; the germinal vesicle is not defined by a limiting membrane ; within the vitelline membrane, which is still delicate and structureless, is a well-defined zona radiata with closely arranged vertical pores. In the third stage the external vitelline membrane is much thicker and shows a radial striatum, the pores being further apart than those of the external zona radiata ; the latter is commencing to disappear. The formation of yolk is in active progress and takes place within the ovum, though not necessarily in its peripheral layers. In the more mature ova the yolk occupies the whole extent of the ovum ; occasionally a peripheral layer is to be distinguished from a central mass ; the vitelline membrane alone is present, and is excessively thin and apparently homogeneous, with no trace of radial striation. The germinal vesicle has undergone certain changes, chiefly in the centripetal movement of the germinal spots. A single section frequently contained ova of all the above mentioned stages ; besides these there were present in the ovary a vast number of ova which undergo a different development; in size these ova were generally larger than those of Stages 2 and 3. The follicular epithelium of the ova is composed of a single layer of large, more or less columnar cells filled with granules exactly like the yolk-particles ; these cells proliferate and migrate into the interior of the ovum ; eventually they disappear: the yolk of these ova appears to be, at least in a great measure, formed by these cells, whose contents are round granules quite indistinguishable from the yolk. The follicular epithelium rests directly upon the ovum and is separated from it by no membrane. The earlier stages of these ova seem to indicate that they are formed by the fusion of a number of germinal cells. At a later stage these ova were indistinguishable from other mature ova ; the follicular epithelium decreases in importance, and a delicate membrane is formed between it and the ovum. |