OF THE CHORDA DORSALIS. 13 



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 sub- 

 stance which refracts the light in a different manner from the 

 cell-walls, just as it is represented in plate I, fig. 1 c, 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 larvae is not great, at 

 least in such as are to be had in the latter part of autumn. Id 

 the above-mentioned species of Cvprinus, 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 larvse 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. (PI. 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 larvse 

 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 belonsr to 

 a cell lying above or below them. Caution must also be 

 used, not to confound a globular epithelial cell, which may have 

 slipped into the chorda dorsalis in making the transverse sec- 

 tion, with the true cells of that structure. I have not as vet 

 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 



