14 STRUCTURE AND GROWTH 
the figure,) lay in the inner surface of the young cell. It must 
remain a question whether the nucleus is really wanting, or 
whether it is only not visible in consequence of its translucency, 
or whether these corpuscles are developed into the nucleus. 
The chorda dorsalis accords with the vegetable cells, at least in 
this respect, that young cells are formed within the old ones. 
With regard to the thickening of the cell walls; these ap- 
pear to remain always simple (unchanged) in the chorda dorsalis 
of the larva of the frog. But im the fully developed osseous 
fishes, in Cyprinus, for example, a thickening is exhibited in 
those cells which le near the axis of the conical interspaces 
of the vertebre. The cell-cavities always become smaller in 
consequence of this thickening of the walls. The thickened 
walls, or the intermediate substance between the cell cavities 
consist of closely cohering longitudinal fibres, between which 
very fine transverse fibres are also sometimes seen. The longi- 
tudinal fibres run uninterruptedly past several cells; and the 
primitive membrane of each cell can no longer be distinguished. 
To sum up the researches upon the chorda dorsalis in a few 
words ; it may be said to consist of polyhedral cells, which have, 
in or on the internal surface of their walls, a structure, according 
in its form and position with the nucleus of the cells of plants, 
namely, an oval flat disc containing one, two, or more rarely 
three nucleoli. The cells usually le in close contact with each 
other; but sometimes at points where three or more cells meet 
together, a sort of intercellular substance, or an intercellular 
passage is seen. Young cells, which are at first round, and float 
free, are formed within parent cells. Nuclei of the charac- 
teristic form, are not distinctly observed in these young cells, 
but sometimes a small globule lies upon their inner surface. In 
those cells which undergo further development, the cell-mem- 
brane ceases to exist as a distinct structure, and the interme- 
diate substance between the cell cavities consists for the most 
part of longitudinal fibres. 
With the exception of the formation of these fibres, into the 
origin of which I have not yet examined, and the absence of the 
nucleus in the young cells, these cells entirely accord with the 
vegetable cells. It must remain undecided whether the nu- 
cleus is really wanting in these young cells, as it is not yet 
proved to exist in all plants, (for example in many acotyledo- 
