OF THE CHORDA DORSALIS
and between the cells a small triangular interstice is seen, which is filled by a transparent fluid (not by air), or at least by a substance which refracts the light in a different manner from the cell-walls, just as it is represented in plate I, fig. 1c, from the onion.
Young cells, which float free, form within the cells of the chorda dorsalis, as in plants. They are, however, in the larvae of the frog so transparent, that very favorable light and good instruments are required to see them. The number of cells, also, in which new ones are formed in the larve is not great, at least in such as are to be had in the latter part of autumn. In the above-mentioned species of Cyprinus, and also in other fishes, they are, however, easy to be seen, and in greater number. Vesicles of very various sizes may be perceived in the cavities of many of these cells, and also in those of the larve of the frog, though they are more difficult of observation in the latter ; a single one of these vesicles sometimes fills the greater part of the cavity; and occasionally several lie in one cell. (Pl. I, fig. 4, b, b, c.) They are commonly quite round; but not unfrequently two are in contact, and flattened against each other. That they lie free in the cell, follows from the fact, that they may be isolated without rupture. If, for instance, a small portion of the chorda dorsalis be torn into minute pieces, and a thin plate of glass with water be placed upon them, by moving this lightly backwards and forwards a few times, some such isolated vesicles may often be brought into the field of vision. They may then be made to roll about, and thus demonstrate their globular form. I have taken great pains to discover a nucleus in their walls, but without suc- cess. The young cells of the chorda dorsalis, also, in the larvae so often mentioned, have often the appearance, so long as they are not isolated, of possessing a nucleus: but one may readily be deceived here, since such a nucleus may belong to a cell lying above or below them. Caution must also be used, not to confound a globular epithelial cell, which may have shipped into the chorda dorsalis in making the transverse section, with the true cells of that structure. I have not as yet been able, with certainty, to observe any nucleus, at least not of the characteristic form, in isolated young cells of the chorda dorsalis. In rare instances, a very small corpuscle, (d, d, of
