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 in the fully developed osseous 

 fishes, in Cyprinus, for example, a thickening is exhibited in 

 those cells which lie near the axis of the conical interspaces 

 of the vertebrae. 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 lie 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 farther 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- 



