62 M. L. Shorey 
removed, so that there is a considerable loss in the amount of 
musculature, the nerve trunks, ganglia, and that portion of the 
spinal cord in direct communication with the injured area are 
defective. 
c The extent of the defect depends both on the amount of 
musculature destroyed and the degree of differentiation of the 
spinal cord at the time when the larva is preserved. In some 
instances a number of branches of the nerve trunk are entirely 
wanting. 
d ‘There is no appreciable effect on the medullary tube until 
differentiation is apparent, but the effect always appears before the 
development of the ventral horn and ventral roots is complete. 
e Nonerve fibers are found wandering free in the mesenchyme, 
but they always follow a well-defined path leading to a muscle or 
other end organ. 
2 Ifthe greater portion of a somite or somites is removed from 
a larva of Rana pipiens shortly before hatching, regeneration occurs 
but the amount of musculature is defective, and the nervous 
system is affected in the same way as that of the amblystoma or 
the frog when a limb bud is removed. 
3 In any of these cases it is impossible to demonstrate depenen™ 
ating nerve cells or nerve fibers, and the defects must therefore be 
due to the failures of these to develop. 
G Conclusions 
Comparing the data obtained from experiments on amphibians 
with that obtained from the study of the chick embryo, it is evi- 
dent that they agree in all essentials; in both, the destruction of a 
peripheral area leads to a defect in the nervous system, which is 
due, not to degeneration, but to the failure of certain neuroblasts 
to differentiate normally. The difference in the extent of the 
defect and the length of time required for its appearance are obvi- 
ously due to regeneration, and to the slow rate of differentiation of 
the nervous system in amphibians. And we are again forced to 
conclude, either that there are two classes of neuroblasts, one of 
which is self-differentiating and the other not, or that these which 
do develop normally are stimulated to do so by the presence of end 
