16 ALBERT KUNTZ 
aspect of the body when the neural tube was closed anteriorly, 
but still open posteriorly... This embryo was placed in contact 
with another which had been subjected to the same operative 
procedure in such a manner that the cut surfaces of the neural 
tubes were approximated. The embryos healed together in 
this position, thus preventing any regeneration of the dorsal 
portions of the neural tubes. They were killed three and one- 
half days later. This is essentially a repetition of Harrison’s 
experiments, using embryos of another species (probably Rana 
pipiens). As observed in the figure, the fibers of the ventral 
nerve-root are not naked, but are accompanied by cells obviously 
of medullary origin (fig. 8, mc). A similar result is illustrated 
in figure 9, which is taken from a section of an embryo of the 
frog (6) which was subjected to the same operative procedure as 
the preceding one, but was not grafted to another embryo. It 
was killed three days after operation. As observed in the figure 
the remnant of the neural tube is somewhat asymmetrical, but 
spinal ganglia and dorsal nerve-roots are absent. 
If the writer correctly interprets Harrison’s present point of 
view, he does not maintain that no cells of medullary origin 
advance peripherally along the fibers of the ventral roots of the 
spinal nerves in embryos of the frog, but that such migration 
occurs relatively late in embryos of the species which he studied. 
In presenting the observations recorded above, there is no dis- 
position on the part of the writer to criticise the work of Harri- 
son, but to point out that in embryos of at least one amphibian 
species, cells of medullary origin migrate into the ventral nerve- 
roots early. As pointed out above, some of these cells advance 
into the sympathetic primordia. The evidence presented in 
figures 8 and 9 does not demonstrate that any of the cells ac- 
companying the fibers of the ventral nerve-roots become incor- 
porated in the neurilemma. However, the availability of these 
cellsisdemonstrated. Furthermore, a careful study of a consider- 
able number of embryos of the frog in which the ventral roots 
of the spinal nerves are present, but the spinal ganglia and dor- 
sal nerve-roots are absent, failed to reveal spinal nerve-fibers 
unaccompanied by cells of nervous origin. Such cells are pres- 
