Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/71

GERMINAL MEMBRANE. germ-vesicle. Before the spot can be considered to be the nucleus, it is necessary that it should, in the first instance at least, be connected with the wall of the vesicle. If, however, the germinal vesicle be the nucleus of the yelk-cell, it is essential, in the first place, that it should, in all probability, be present before the yelk-cell; at all events, that in proportion as the ovum is younger, should the vesicle be larger in relation to the cell; 2dly, it must, at first, he upon the vitelline- membrane, and be more or less intimately connected with it; 3dly, the germinal-vesicle, when regarded as a nucleus, either has no nucleoli, or Wagner’s spots are to be considered to represent them; in the first case they form the contents of the nucleus. In the enumeration of these points, no regard is had to the relations of the germ-vesicle subsequent to impregnation, because it is desirable to determine its ultimate destiny, to a certain extent a priori, from its signification, and thus to be enabled at the least to afford a guide to the much moré difficult observation of the fecundated ovum. If the researches were complete, the distinctions above cited would be sufficient for the correct determination of the question at issue, the decision of the first point indeed would of itseif be ample evidence.

When we take into consideration the first point raised on either side, we should be compelled to decide in favour of the latter view, and regard the germ-vesicle as a nucleus, if it were proved to be first present, and also that the yelk-cell is formed around it as a simple cell, narrowly encompassing it in the first instance, and becoming gradually expanded. In the next place, it is certain that at an early period the germ-vesicle is much larger in proportion to the yelk-cell, and that it at first grows pari passu with the yelk-cell, but that subsequently the latter increases in size in a much greater ratio, whilst the vesicle remains stationary; and these are precisely the relations in which the vesicle should stand in order to be regarded as a nucleus. But these facts are not entirely irreconcilable with the first view. A young cell, the germ-vesicle, might be imagined to form within the yelk-cell at a very early period of its growth, which young cell might at first increase in size more rapidly than the original one, but cease to do so earlier, whilst the parent-cell might continue to be developed