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 appear 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 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 longitudinal 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 characteristic 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-membrane ceases to exist as a distinct structure, and the intermediate 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 nucleus is really wanting in these young cells, as it is not yet proved to exist in all plants, (for example in many acotyledo-
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