38 M. L. Shorey 
leg presents some new features. This area is developed from 
6 somites, the 20th to the 25th inclusive, and each of the 6 nerve 
trunks innervating it has practically the same peripheral distri- 
bution. The course of these nerves is shown in Fig. 37, from 
which it will be seen that the nerve divides at once into a dorsal, a 
lateral, anda ventral branch. In this specimen the parts inner- 
vated by the dorsal and ventral branches are normal, but there are 
only remnants of the area to which the lateral branch is normally 
distributed, and in some cases it is entirely absent. From the 
conditions found in the brachial region it would be expected that 
the lateral nerve would be smaller or wanting, and this was found to 
betrue. But the nervous system is further affected, for instead of 
6 ganglia and nerve trunks, there are only 4. It is impossible to 
say which ganglia are missing, for this result is brought about 
partly by an increase in the thickness of those present, and partly by 
a greater distance intervening between successive ganglia. While 
the loss of the 2 ganglia is compensated for in some degree by the 
increased thickness, the area of the cross-sections of any ganglion 
is always less, and the nerves are decreased in size. As the begin- 
ning of both the ganglia and the peripheral nerves must have been 
established before the operation, it seems difficult to make it 
account for this condition. But as it occurred again in another 
ectopic embryo and only in this, it can hardly be regarded as a 
mere coincidence. Moreover, a younger embryo, to be described 
later, gives a clue to its meaning. 
Another noticeable feature is the effect on the spinal cord 
(Fig. 17). There is no antero-lateral projection of the ventral 
horn in this region, and no definitely localized area of loss can be 
demonstrated, but the whole half is decreased in size while retain- 
ing its normal shape and proportions. 
Examination of these three cases demonstrate clearly that as 
early as 5 days after the extirpation of peripheral areas well defined 
defects appear in the nervous system, but consideration of this 
period alone can give no conclusive evidence in regard to the 
process by which this condition was reached. Logically any one 
of three reasons might be assigned: 
1 The injury inflicted has caused this portion of the embryo to 
