Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/69

GERMINAL MEMBRANE.

membrane, the double outline of which he recognised, (plate II, fig. 1, d.) It is thus highly probable that the yelk of the mammalian ovum is a cell. Even if, as Wagner intimates, the vitelline membrane in other animals should sometimes be formed only secondarily within the chorion, it would not materially interfere with our purpose, since in that case the chorion would be the cell-membrane. The ovum universally possesses an external closed membrane (whether it be chorion or vitelline membrane), which is structureless, and not gene- rated from other elementary structures, and therefore is the ovum always a cell. The yelk-cell encloses the vitelline substance as its cell-contents, and upon its internal surface lies the germinal vesicle, or vesicle of Purkinje, (fig. 1, f) This, as is known, is a very transparent thin-walled vesicle, containing a pellucid fluid, according to Wagner coagulable by spirits of wine. It encloses almost universally (Wagner cites but very few exceptions) upon the internal surface of its wall, a corpuscle, called by the discoverer, R. Wagner, germinal spot, or germinal disc, (fig. 1, g.) In mammalia it is generally flat. In many instances several of these spots are present, their number, however, is said by Wagner to bear proportion to the age of the ovum, they being fewer and much more firmly attached to the wall of the germinal vesicle in young ova, I have frequently observed in osseous fishes (where they are often pre- sent in such numbers as to prevent the fluid in the vesicle from being seen) that when one of these corpuscles, after the bursting of the germ-vesicle, passed through a narrow space, it first became considerably elongated, and then drawn out in the centre to a thin thread, which soon broke. The two ends afterwards retracted, and thus two round globules were produced from one corpuscle, in a similar manner to what we may observe in the drops of fat upon soup. They appear, therefore, to be composed of a tenacious substance which is not miscible with water. Purkinje states that the germinal vesicle in birds is firmly fixed to the vitelline membrane, but Baer and Wagner describe it as lying in the centre of the yelk at first, and rising to the surface at a subsequent period.

The decision of the question, as to the precise signification of the germinal vesicle, now becomes of great importance. Is it a young cell generated within the yelk-cell, or is it the