Ross Granville Harrison 127 
normal specimens. In two cases motor as well as sensory nerves were 
found; once in one, and once in two segments, though in other segments 
only sensory nerves were present. Here the motor nuclei had been less 
completely removed than in the other cases. In one case neither 
sensory nor motor branches were found. The last named case is to be 
explained as due to imperfect union of the parts, as is also the fact that 
in other cases the sensory nerves were often scantily developed. 
The results of these experiments show, therefore, that during the 
period in which the specimens were kept under observation, the sheath 
cells are unable by themselves to form nerve fibers. The negative 
character of the result renders it necessary, however, to secure further 
cases before this conclusion can be regarded as established beyond all 
question. Should it be urged that time enough was not given the sheath 
cells to form the nerves, it may be pointed out that in normal specimens 
the motor fibers develop at a much earlier stage and that if the sheath 
cells normally contribute to their formation they should unquestionably 
act in the period allotted. The purpose of the experiment was to deter- 
mine the normal behavior of the cells and not any possible regulative 
action on thir part, which might take place later. 
The above experiments deal only with the motor nerves, and it has 
not been found practicable to experiment systematically with the sensory 
nerves because in the latter the ganglion and the sheath cells have a 
common place of origin. In studying the normal development of the 
sensory nerves in the amphibian embryos, we find important evidence 
bearing upon the question. For instance, the nerves derived from the 
dorsal (giant) cells of Rohon-Beard are formed without sheath cells. 
These fibers consist, in fact, of naked axis cylinders, which branch and 
form a delicate plexus of nerves under the skin of the frog larva, and are 
entirely devoid of cells (or nuclei). Again in the Triton larva, even some 
of the nerves derived from the spinal ganglia of the tail are for a short 
time devoid of sheath cells; these, together with the nerves from the 
dorsal cells form a non-cellular plexus in the fin folds.” In the frog 
larva the nerves derived from the spinal ganglia have sheath cells 
from the beginning. Comparison of these instances show that these 
cells are a variable element in the young nerve fiber; it may 
therefore, be concluded that they play no necessary part in 
6 Since this fact was disputed by O. Schultze (Archiv f. mikrosk. Anat., Bd. 
66, 1905, p. 68), I have again examined the specimens in question and have 
nothing to correct in my former statement. It may be added, however, that 
I did not intend to include the n. lateralis, which is independent of the 
cutaneous plexus. This nerve of course has sheath cells at this stage. 
