Differentiation of Neuroblasts 51 
24. hours after the operation, its extent and character depending 
on the extent of injury to the periphery. If only the brachial 
musculature is removed no effect manifests itself at this time except 
a slight decrease in the extent of the peripheral branching, and a 
more doubtful decrease in the size of the ventral horn. If all the 
musculature is destroyed no definite nerve trunks are formed, but 
the fibers wander free in the mesenchyme and the ventral horn is 
more defective. 
14 The effect of operations performed on the second or third 
day differs only in the amount of musculature involved. 
15 In operations at a later period, when connection with the 
end organs has already been made, and therefore involve section- 
ing a nerve already formed, degeneration may occur. ‘This starts 
at the peripheral end of the nerve and extends slowly toward the 
center, so that the spinal cord is affected at a much later stage 
than it is when the operation causes inhibition of growth. 
16 In operations performed on the fourth day, since no degener- 
ation occurs when the destruction of the musculature is not com- 
plete, the nerves already formed and sectioned by the operation 
would seem to be directed to the peripheral organs which are not 
destroyed. 
17 No degenerating fibers can be observed when the extirpated 
areas are removed before the nerves reach the region of the opera- 
tion, and in no specimen studied has degeneration reached the 
nerve cells; so that it may be added as the final demonstrable fact 
that— 
18 Under the conditions of the experiment, the defects which 
appear in the nervous system are not due to degeneration, but to 
the failure of the neuroblasts to develop. 
HN. Discussion 
In discussing the bearing of these facts on the problem of the 
differentiation of the neuroblasts, I shall limit myself, for the sake 
of clearness, to a consideration of motor neurones, for the extent 
of an injury to the musculature can be more sharply defined than 
an injury to sensory areas, and the process of differentiation of the 
motor neuroblasts can be more easily followed. 
