Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/82

THE OVUM AND within the yelk in the first instance, and in each of these again two new ones, and so on.

We next proceed to consider the changes undergone by the external layer of cells furnished with nuclei. In eggs which have a diameter of a line, this entire membrane, if it may be so called, appears to be made up merely of cells. In such as have reached a higher stage of development, such as have a diameter of upwards of half an inch, for instance, it consists of two strata, the external of which is granulous, and no longer exhibits cells ; the internal, however, is composed of cells, which are flat, hexagonal, but also granulous, and bear the relation of a covering of epithelium to the outer one. The external stratum passes away over the germinal vesicle and the foundation of the germinal membrane, so that these structures may easily be removed from its inner surface without injury to it. The internal cellular stratum, on the contrary, is interrupted at the spot where the germinal vesicle lies. I have not traced the mode of formation of this external granulous stratum through all its details; I suppose it to be produced by a blending of the outer cells, which composed the original membrane when it was made up entirely of cells. As the period approaches at which the egg leaves the ovary, the epithelium-like stratum of cells gradually disappears, and the granulous membrane alone remains. It does not exhibit any disposition to unite with the structureless external membrane of the egg, even in eggs which are almost sufficiently mature for extrusion. If such an egg be cut open under water, and the investment derived from the ovary be drawn off, this granulous membrane frequently remains lying upon the yelk, whilst the structureless membrane follows the above-mentioned investment, and may readily be proved to be connected with it, when they are folded so that the inner surface forms a sharp edge. By the aid of the compressorium this structureless membrane may then be seen, projecting out from the border of the preparation. It often separates in large pieces during this manipulation, so that it has likewise no connexion with the parts pertaining to the ovary. If the signification of vitelline membrane is to be assigned to this structure, a blending between it and the granulous stratum must take place in the oviduct, in order to form the subsequent vitelline membrane of the extruded egg.

We now pass on to that portion of the egg from which the