Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/86

THE OVUM AND layer of the germinal membrane of an egg after sixteen hours’ incubation, we find it to be composed of globules, which vary greatly both in size and appearance, (see plate II, fig. 7.) The large globules, which form the greater proportion, may be proved to be cells, and Baer has already named them vesicles. The molecular motion, which is frequently visible in isolated globules within them, although much slighter in these instances than in the cells of the serous layer, affords sufficient evidence of their cellular character. They contain a transparent fluid and granules of various kinds. One particular globule, having very dark outlines, resembling those remarked in the cells of the yelk-cavity, may be observed in almost every cell. Several of the globules, and of all gradations of size, are frequently seen in a cell. In addition to the above, a minutely granulated substance is present in many of them. These cells lie somewhat loosely together in a structureless, tenacious, intercellular substance, which is their cytoblastema, so that at this stage they are but slightly flattened against one another. This intercellular substance contains, in addition, perfectly dark globules and smaller granules, but I do not know what relation they bear to the cells. A portion of them may, perhaps, be nuclei of new cells. Yet I could not decide whether the one dark globule, which is generally so very prominent in the cells of the mucous layer, had actually the signification of a cell-nucleus. It differs in form from the usual cell-nucleus very materially. During the progressive development of the germinal membrane, the quantity of intercellular substance, and of those globules the cellular nature of which is not demonstrable, diminishes very much, so that at a subsequent period the cells lie close together, and present the appearance of vegetable cellular tissue. The description here given applies only to the mucous layer on the outside of the area pellucida. Within that the cells have quite a different appearance. They are very much smaller, of pretty equal size, very transparent, and contain no coarse granules, but only very small globules. They do not appear to have any nucleus, and this fact distinguishes them from the cells of the serous layer, which possess a nucleus even within the area pellucida.

The first rudiments of the embryo appear to be formed from the cells of the serous and mucous layers of the germinal mem-