Differentiation of Neuroblasts 47 
directly destroyed. ‘This embryo, therefore, offered a favorable 
opportunity to study the behavior of the motor nerves alone. 
On the normal side the nerve trunk, though smaller than when 
the sensory root is also present, has reached the inner margin of 
the wing, the typical condition at this stage of development. (Fig. 
2.) The fibers on this side follow a straight course and form a 
compact mass as usual. On the operated side, the ventral horn 
is much smaller but some fibers are formed; these, instead of 
following a straight path through the musculature to the region 
where the inner margin of the wing would normally be found, end 
in an irregular mass a short distance from the medullary tube. 
(The same condition was found in Experiment 69 and is repre- 
sented in Fig. 40.) 
Experiment 69. In this specimen, preserved 24 hours after an 
attempt to remove the three brachial somites, there were two eleva- 
tions in the region of the wing with a depression between them, but 
these were together much smaller than the normal wing. On 
sectioning I found that the myotome of the first brachial somite 
was entirely missing, but there was some lateral outgrowth of 
tissue to form the wing. In the second somite there was no myo- 
tome, and practically no thickening of the wall in the wing region. 
In the last somite both the myotome and the wing bud were nearly 
normal. ‘There had again been a very slight injury to the spinal 
cord in the most dorsal portion and as the brachial ganglia were 
entirely wanting it is probable that the neural crest was destroyed 
by the operation. 
The conditions in the second brachial somite were practically 
identical with those described in the last experiment with the possi- 
ble exception that a slight thickening of the body wall may contain 
a very small amount of muscle tissue, not yet sufficiently differen- 
tiated to be identified. But as there is no evidence of the myo- 
tome, which should be prominent at this period, and the nerve 
fibers behave exactly as in Experiment 84, it seems probable that 
absolutely all of the primordium of the muscle cells was destroyed. 
The ventral portion of the spinal cord and the path of the nerves 
on the operated side are represented in Fig. 40. The nerve trunk 
on the operated side follows the normal course in direction and 
extent, but again lacks the sensory fibers. 
