Differentiation of Neuroblasts 49 
the operated side, as is also the ganglion, but the nerve is not notice- 
ably smaller, due presumably to the addition of fibers from the 
somite above. Most of this nerve passes down to the next somite 
as the sympathetic branch, which is much larger on this side than 
it is on the normal side. Its course is shown in Fig. 42, B. The 
effect on the spinal cord is illustrated by Fig. 21, a cross-section in 
the region of the 15th nerve. 
In this process of the union of two ganglia and nerve roots is to 
be found an explanation of the condition described in Experiments 
225 and 179, where, in the injured region, there were but four gan- 
glia and nerves instead of six, whileno one region of thespinal cord 
was more defective than another. As was shown in experiments 
69 and 84, some nerve fibers do develop in every somite even when 
all the end organs of the motor cells are destroyed and the sen- 
sory area much decreased. (It is impossible to have a region in 
which there may not be some sensory tissue.) These fibers at 
first wander freely in the mesenchyme without forming a compact 
nerve, but are later presumably attracted in the direction of the 
end organs above or below, and finally the ganglia and nerve roots 
unite. ‘This process had been completed in Experiments 225 and 
179, without leaving any direct evidence of its having occurred. 
G. Summary 
1 The complete destruction of the primordium of any muscle 
before its innervation results in the complete suppression of the 
branch of the peripheral nerve leading to it. 
2 Complete destruction of all the muscles of a given somite 
does not, however, lead to the complete suppression of all the motor 
cells and motor fibers at this level of the spinal cord, but the num- 
ber is very greatly reduced, and these present innervate muscles 
of the somite above or below. 
3. The nerve leading to a defective muscle resulting from the 
destruction of a portion of its primordium before innervation is 
always decreased in size, but it is usually larger 1 in proportion to 
the size of the muscle than the normal nerve is in proportion to the 
size of the normal muscle. This holds true even if the most 
minute fragment of the muscle is left, and in general it may be 
