Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/38

STRUCTURE AND GROWTH be more accurately examined, it is found that the granules are oval, and furnished with a nucleolus, and that, with the exception of their being only about half as large, they entirely resemble the cell-nuclei. This cortical substance is not sharply separated from the proper tissue of the chorda dorsalis; and as the cells of the latter suddenly diminish very much towards the cortical substance, I think that these granules upon the latter are the cytoblasts of flattened cells which form it. Sometimes, although but indistinctly even with a very favorable light, very fine lines may be perceived in the intermediate spaces between these granules, where the cells come in contact, as in the common tabular (or scaly) epithelium. In the chorda dorsalis of the larva of Rana esculenta, where the nuclei in the cells are not distinct, these nuclei in the cortical substance are not seen; the tabular structure, however, is evident in them. One must be very cautious in denying the presence of the cytoblasts, when they are not immediately recognizable. They may in animals, as in plants, attain such a degree of transparency, as renders them very difficult of observation. Thus, I could not for a long time detect them in the rudiment of the chorda dorsalis, which is found in the conical intermediate spaces of the vertebra, in a large Carp, until on a very clear day they appeared very pale but quite recognizable, and of precisely the form above described. They were somewhat more distinct in the Pike and Cyprinus erythrophthalmus. The delineation, plate I, fig. 4, is taken from the latter. They are however smaller in these fishes than in frog’s larvee.

To return to the larva of Pelobates fuscus. Here the cells of the chorda dorsalis lie so close to each other, that the walls of the two neighbouring cells are in immediate contact. Even when three or more cells are in contact, they are generally so close, that only the contiguous walls are observable. Sometimes, however, in such instances, a small intermediate space remains, which is larger than could be filled up by the unthickened cell-wall; and there is then seen, as in plants, a species (apparent or real?) of intercellular substance, or an intercellular canal. With regard to this latter (intercellular canal), occasionally, though rarely, in such an instance of close contiguity of three cells, upon making a transverse section, the cell-walls are observed sharply bounded, as well towards the cell as externally,