Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/37

OF THE CHORDA DORSALIS. usually largest in the centre, and becoming somewhat smaller towards the outside. They have an irregular polyhedral shape, mostly with spherical surfaces, which are sometimes convex towards the outside, sometimes towards the cavity of the cell. Their walls are very thin, colourless, smooth, and almost completely transparent, firm, and slightly extensible. They dissolve readily in caustic potash. The rudiments of the chorda dorsalis in the conical interstices of the vertebrae of cartilaginous fishes are not dissolved by dilute or concentrated acetic acid. The chorda dorsalis of fishes according to J. Müller does not become converted into gelatine after long boiling.

The cells of the chorda dorsalis of frog’s larvee contain in their interior a colourless, homogeneous, transparent fluid, which does not become cloudy at a boiling heat; the slight clouding observed in the chorda dorsalis after boiling, appears to be situated more in the cell-walls, which afterwards appear minutely granulated.

In the larva of Pelobates fuscus another formation occurs, inasmuch as by far the greater proportion of these cells containv a very distinct nucleus. It has the appearance of a somewhat yellowish-coloured small disc, of a roundish oval form, rather smaller than a blood-corpuscle of the frog, and almost as flat. (See plate I, fig. 4a, where it is represented from the chorda dorsalis of Cyprinus erythrophthalmus.) In frog’s larvae the nucleus is nearly twice as large. It has a sharp, dark margin, and appears minutely granulated. In this little disc may be seen one, rarely two, and very seldom three dark, sharply circumscribed spots. It thus entirely resembles, both as a whole as well as in its modifications, the cytoblast of vegetable cells with its nucleolus, and microscopically, cannot at all be distinguished from it. Compare plate I, fig. 4a, with plate I, fig. 1a. But it also corresponds with it in its position in the cell. In very many cells, the vertical wall of which is viewed from above, it may be seen that the nucleus lies close on the inner surface of the wall of the cell, or even embedded in the wall. It appears then, as in plate I, fig. 1 a, only still somewhat flatter. I have not, however, succeeded in observing that a lamella of the cell-wall passes over its internal surface, which is also but rarely seen in plants. If the external minutely granulated cortical substance of the chorda dorsalis of Pelobates fuscus