Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/68

THE OVUM AND by a very delicate pellicle, the albumen-membrane, b, also that the transparent substance itself (albumen) is sufficiently fluid to permit of such a degree of displacement of the yelk as to allow of its coming into contact even with the albumen-membrane. Although I have never yet succeeded in observing this pellicle, and though in my researches the transparent membrane, on the bursting of the yelk, always tore with smooth edges like a solid substance, yet the observations of the respected discoverer are too precise to admit of a doubt upon it. It is also supported by the analogy of most of the ova of other classes of animals, in which chorion and vitelline membrane may generally be distinguished, notwithstanding that they sometimes lie close upon each other. The albumen-membrane has probably the signification of a cell-membrane, in which case the albumen will be the cell-contents, and the yelk a young cell. According to Wharton Jones, the transparent areola (zona pellucida) of the ovum, or the albuminous layer in the fecundated ovum of mammalia, becomes considerably expanded in the tubes, a fact which would be readily explained by the inherent energy of the albumen-membrane when regarded as a cell. In such case, however, the mode of formation of the albumen would be very different from the corresponding process in the bird’s egg, where, according to Purkinje, it is secreted by the oviduct, and a membrane (chorion) is formed around it subsequently, which cannot therefore have the signification of a cell-membrane, and is moreover not simple in structure, but composed of fibres. Meanwhile an investigation might be made, as to whether the albumen in the egg may not also be first surrounded and formed by an equally thin pellicle, around which a secondary external membrane may subsequently be produced. According to Purkinje, however, this is not the case, and I could not discover any such pellicle upon the inner surface of the shell-membrane of the excluded egg. I have not made any inquiry as to whether the chorion of fishes is a cell-membrane or not. It is covered internally with a very beautiful epithelium, which is made up of more or less flat hexagonal cells, each of which has its nucleus.

Within the transparent areola, or, according to Krause, the albuminous layer, lies the vesicle of Baer, or the yelk ; which, from Krause’s statement, is enclosed by a peculiar structureless