52 M. L. Shorey 
So far as the process of differentiation of a motor neurone can 
be observed, it consists in the movement of the neuroblast into 
the region of the ventral horn, increase in its size and change in 
the shape of the cell itself, and the development of a fiber. Every 
motor cell which is entirely self-differentiating would, under the 
conditions of the experiment, reach the ventral horn and develop a 
fiber, for this region is uninjured and the food supply unaffected. 
But as in some cases the motor nucleus and motor roots are not 
more than 25 per cent of their normal size, and as it has been shown 
that this is not due to degeneration but to failure to develop, it is 
clear that at least 75 per cent of the motor cells are influenced in 
their development by the presence or absence of the muscles 
which they normally innervate. Moreover, as 3 or 4 days after the 
operation there are more undifferentiated neuroblasts on the in- 
jured than on the uninjured side, and the cells which are found 
in the ventral horn are perfectly normal, it seems equally apparent 
that even the initial stages of differentiation are wanting, and that 
these cells are entirely dependent on the presence of their muscular 
end organs for differentiation. 
But in every specimen examined, even when there is no trace 
of musculature in the somite concerned, some motor cells do 
differentiate completely. We are then forced to one of two con- 
clusions :—either we have two classes of motor neuroblasts, one of 
which is entirely dependent on stimulation from the periphery for 
differentiation, and the other entirely independent of such stimu- 
lation; or the neuroblasts which do develop normally are influenced 
by the musculature of adjacent somites. It seems improbable 
on purely a prior: grounds that such a wide difference inthe phys- 
iological activity of cells having a similar origin, and normally a 
similar destiny, as the first assumption postulates, can exist. But 
if it is to be assumed that the muscles of other somites are a source 
of stimulation, it is perhaps necessary to show that the nature of 
the stimulus may be such that it would naturally be expected to 
extend to other regions of the spinal cord than that from which the 
nerves innervating these muscles normally arise. This I shall 
attempt to do by considering some of the factors involved in differ- 
entiation. 
