Differentiation of Neuroblasts 59 
sides can be detected, and even a few in which the motor nucleus 
is larger on the injured side. On the whole, however, there is no 
doubt but that the ventral horn of the normal side is larger. The 
brachial plexus is also somewhat decreased in size. Aside from 
this specimen, I have no other in which the nervous system is 
noticeably affected until 14 days after the removal of the limb. 
The following experiment will serve to illustrate this condition: 
Experiment 27. The forelimb, in which the beginning of the 
formation of two toes could just be detected, was removed May 3, 
and the specimen was preserved May 17. The animal grew slowly 
and regeneration was correspondingly slow, but evident. At the 
time of preservation the nerves have entered the limbs, but are still 
little developed, the main trunk being but .o3 mm. in diameter in 
the widest part. ‘[he motor roots are very minute, and the motor 
horn is just beginning to be perceptible as a distinct unit. Figs. 
28 and 29 are taken from a cross-section in this region. The defect 
shown in the spinal cord is rather extreme, the difference between 
the two sides not being quite so great in most cases. 
I have no specimen younger than this, except the one just 
cited, in which the nervous system is not developing normally, and 
I have older specimens in which it is also normal. But in all of 
these cases some of the limb muscles are present, and regeneration 
is going on, so that the amount of loss in the mass of the muscula- 
ture is comparatively small, and the point I wish to establish is 
that when the peripheral area is extensively injured a defect appears 
in the nervous system before differentiation is completed. Com- 
paring the nervous system of the normal side of these larvae which 
were killed respectively, 10, 14, and 33 days after the removal of 
the limb, it is evident that there are less fibers in the nerve trunks 
and nerve roots, and less cells in the motor horn of the two former. 
Since a defect has already appeared, there can be no time at which 
all the neuroblasts which normally differentiate into motor nerves 
are found together in the motor horn. 
Have the missing motor cells been formed and then degenerated ? 
To answer this I shall compare the larva in Experiment 27 with 
another of the same age, kept under the same conditions, and 
apparently developing parallel with it, but which was killed nine 
