794 
ROSS GRANVILLE HARRISON 
embryo is not, in my opinion, certain, but in sections of preserved 
specimens, as Held has described in great detail, a delicate net- 
work, is visible. The character of this intercellular reticulum 
varies from specimen to specimen and, as will be seen, varies very 
greatly according to the mode of preservation. It seems to be 
beyond doubt that the structures in question are due in part 
to coagulation, though just to what extent it is not easy to say. 
In order to test the matter a series of embryos were preserved in 
osmic acid, which, as Fischer (^01) has shown, fixes protoplasm 
without bringing about any visible change in structure, and which 
after prolonged action (24 hours, 1 per cent, in the case of Amoeba 
proteus) so fixes it that alcohol causes no further change. Sec- 
tions of these embryos show plainly that the spaces between the 
organs described above are almost perfectly clear; only occasion- 
ally do very delicate filaments appear bridging the spaces. The 
contrast with specimens which have been preserved in a corrosive 
sublimate-acetic mixture is very great ; and very much more pro- 
nounced still is the difference shown by embryos preserved in 
Hermann's fluid, which is, however, otherwise a ver\ ill adapted 
preservative for this material.^ It is not intended on the basis of 
the foregoing observations to deny the existence of protoplasmic 
bridges in embryos of this stage, but it does seem proper to call 
attention to the facts just stated, in order to show the necessity 
for caution in ascribing significance to the connection between 
such fine structures and the developing nerve fibers. 
Differentiation of nerve fibers 
The embryo last described is in the stage which was used for 
most of the experiments. It is the oldest stage of which it can 
be said with certainty, without microscopic examination, that 
there are no nerve fibers present. In the next stage to be con- 
® On account of the large amount of yolk, which becomes very brittle after pro- 
longed treatment with osmic acid, the amphibian embryo is not a favorable object 
for the study of this question. It was found necessary to impregnate the embryos 
with celloidin before- embedding in paraffin, and even then the sections were not 
perfectly satisfactory. It would be of great interest to have an exact compari- 
