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, atta 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 mtermediate 
spaces of the vertebra, 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 larve. 
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, 
