12 STRUCTURE AND GROWTH 



be more accurately examined, it is found that the granules are 

 oval, and furnished with a nucleolus, and that, with the excep- 

 tion of their being only about half as large, they entirely re- 

 semble 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 cyto- 

 blasts, when they are not immediately recognizable. They 

 may in animals, as in plants, attain such a degree of trans- 

 parency, 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 vertebrae, in a large Carp, until on a very clear 

 day they appeared very pale but quite recognizable, and of pre- 

 cisely 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 larvae. 



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. Some- 

 times, however, in such instances, a small intermediate space 

 remains, which is larger than could be filled up by the unthick- 

 ened cell-wall ; and there is then seen, as in plants, a species 

 (apparent or real?) of intercellular substance, or an intercel- 

 lular canal. With regard to this latter (intercellular canal), occa- 

 sionally, 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, 



