Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/88

THE OVUM AND which are the subsequent cells of the proper yelk-substance. They are formed round about the neighbourhood of the vitelline membrane, with the exception of that spot where the germ-vesicle and the rudiments of the germinal membrane lie. ‘These cells expand very rapidly, while at the same time a new layer is formed on the outside of them, and so on successively. In this manner they surround the white cells of the yelk-cavity with a layer of yellow cells, which is constantly increasing in thickness ; as, however, a vacant space remains at the spot where the germinal vesicle and germinal membrane are situated, by the increasing thickness of the yelk-substance, the space becomes converted into a canal. The development of the vitelline membrane proceeds continuously with these changes, in pro- portion as the increasing contents require. When the yelk-cell has attained its due size and the egg leaves the ovary, the germ-vesicle, like most other cell-nuclei, disappears, and the now more fully developed germinal membrane remains. It is made up of globules, probably cells, having coarsely-granulated contents. It grows during the process of incubation by the con- tinual development of new cells. After sixteen hours’ incubation, a distinction may be observed in the cells composing the membrane. The more external ones form a layer, in which the cells exhibit a nucleus of the characteristic form, and contain a quantity of transparent fluid and minute isolated granules. These cells are therefore clear, and firmly united together, and have only a minimum of intercellular substance between them; they represent the serous layer of the germinal membrane. The under stratum of the germinal membrane or mucous layer contains cells of another kind; they have no nucleus of the characteristic form, but contain one or more dark globules, and frequently also some minutely granulous substance. These cells lie loosely together in a larger quantity of intercellular substance, which contains smaller granules of different kinds, in addition. When this division of the membrane into the two layers is completed, and its superficies has become considerably extended, and after a transparent spot, the area pellucida, has formed in its centre—(the cells of the mucous layer in this area being much smaller, but of pretty equal size, as compared with one another, and having transparent contents with very minute isolated granules),—the embryo is developed,