Differentiation of Neuroblasts 4I 
to the inner border of the motor nucleus was greater on the oper- 
ated than on the unoperated side. This is shown in Fig. 18, a 
photograph of a section of the spinal cord at the level of the 15th 
nerve. An explanation of this may perhaps be found by assuming 
that the normal number of neuroblasts were formed, but part of 
these were arrested in their development, and never reached their 
normal location in the ventral horn. As before, while the loss in 
the dorsal horn is evident, it is impossible to arrive at any satis- 
factory quantitative estimation of it. 
As mentioned above this embryo was also ectopic, and showed 
the same peculiarity as Experiment 225 in the loss of ganglia and 
nerve roots between the wing and the leg. Unfortunately the 
posterior portion of this specimen was destroyed, and the number 
lost is therefore uncertain, but as far as the series goes there are 
7 post-brachial ganglia on the normal side and § on the abnormal. 
As in Experiment 225 there is an increase both in the thickness of 
the ganglia, and in the number of intervening sections, but the 
dorso-ventral and lateral diameters were taken throughout the 
course of ganglion 16, and from these the average loss in area for 
each section was estimated to be about 25 per cent. Measure- 
ments of the succeeding ganglia were taken at intervals, and 
showed a constant loss; percentages were not reckoned. 
The peripheral distribution of these nerves on the normal side 
agrees with that shown for Experiment 225, while on the injured 
side the lateral branch is missing or defective according as the lat- 
eral musculature is missing or defective. 
Although the spinal cord changes its shape in different parts of 
its course between the wing and the leg, the condition represented 
by Fig. 19 may be taken as typical of the character of th loss. 
This is more difficult to localize than in the brachial region, for the 
motor nucleus is not so clearly defined, but examination with a 
high-power lens shows that less neuroblasts have differentiated into 
the characteristic motor cells. The dorsal horn is also smaller, 
and all the dimensions of the spinal cord decreased. 
Experiment 324. This embryo was preserved 3 days 6 hours 
after the operation and was then 6 days 19 hours old. ‘The amnion 
was grown to the embryo in the region of the wound, and in 
