Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/76

THE OVUM AND usually takes place around the nucleus in other cells; and again, the germ-vesicle disappears, precisely as the nucleus of other cells is generally absorbed. There is then no evidence that the fluid of the germinal vesicle exercises a fructifying influence; but if it be the cell nucleus, it disappears, because it has completed its office,—the formation of the yelk-cell. The dise, which has formed around it, becomes developed into the germinal membrane, and it is uncertain whether the remains of the germ-vesicle also take part in that formation.

We shall next proceed to the consideration of the other contents which the yelk-cell includes in addition to the germ-vesicle, making use of the bird’s egg for the purpose. Setting aside some points of distinction of slighter importance, the globules, well known as present in the yelk of the hen’s egg when laid, may be divided into two principal classes: a, the globules of the yelk-cavity; and b, those of the true yelk-substance. The former (a) are not only present in the yelk-cavity, but occur also in the canal leading from it to the germinal membrane, and in the little prominence, called by Pander the nucleus of the tread (Kern des Hahnentritts). When many of them lie close together, they exhibit a white colour, whilst the true yelk-globules in such circumstances appear yellow. They may also be distinguished from the latter globules under the microscope, (see pl. II, fig. 2.) They are perfectly round globules, with quite smooth edges, each enclosing a smaller one, which is also perfectly spherical, and looks like an oil-globule, being rendered very distinct by its sharp outline.

The remaining space in the large globules is usually transparent, and not granulous. But some may be observed which have granulous contents, and they then completely resemble the true yelk-globules, except that the latter do not generally contain any smaller ones with such dark outlines. Sometimes also, the globules of the yelk-cavity contain two or more such smaller ones. The common yelk-globules (4), that is, those of the true yelk-substance, may be distinguished from the above-described by the following characteristics : they are upon the whole larger, they have all granulous contents, and, for the most part, do not enclose any smaller globules. They are very sensitive to the action of water, which causes them to fall to pieces, and then the granules enclosed within them becoming free, give