Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/70

THE OVUM AND nucleus of the yelk-cell? If the former, it is in all probability the most essential rudiment of the embryo; but if it be the nucleus of the yelk-cell its importance vanishes with the formation of the yelk-cell, and according to the analogy of most cell-nuclei, it must either become absorbed altogether at a subsequent period, or continue for a time simply rudimentary, without forming any important new structure. The following is the ordinary career of a simple cell: a nucleus is present in the first instance; around it a cell is formed; the nucleus at first often increases in size as the cell grows, but their growth is by no means proportionate, that of the cell being much more rapid ; the cell-contents are at first transparent; a firm precipitate or new formation next commences in the cell, and this occurs immediately around the nucleus, which is at first enclosed by it; the nucleus then either becomes entirely absorbed, or continues only rudimentary and (with the following exception) I have never observed it to give origin to any other essential formation. One or more oil-globules once appeared to me to be formed during the absorption of the nucleus in the adipose cells within the cranial cavity of a young carp. The importance of the decision of this question in reference to the germinal vesicle thus becomes very obvious. Unfortunately, however, neither the observations upon the subsequent relations of the germ-vesicle, nor those on the origination of the ovum, are sufficiently extensive or certain for the purpose.

We shall next proceed to analyse both views of the question more minutely, and afterwards compare them with the observations. If the germ-vesicle be a young cell, in the first place, it is absolutely necessary that the yelk-cell should first exist, and that the germ-vesicle should afterwards be developed within it; 2dly, the germ-vesicle must not be connected with the vitelline-membrane, but must be developed free at some chosen spot within the cavity of the yelk; 3dly, the germ-vesicle may be regarded either as a cell without a nucleus, and in that case the spots of Wagner belong to the cell-contents, or Wagner’s spot, when it is single, is the nucleus; when there are several present, the others either differ essentially from one particular spot, and pertai to the cell-contents, or they are nuclei of young cells afterwards to be developed within the