Differentiation of Neuroblasts 29 
should innervate them. ‘The rapidity of development, and the 
availability of material at practically all times of the year were also 
points in favor of the chick. 
B Character of the Operation 
It was decided that the first experiments should be the removal 
of the wing and after some preliminary tests the period at which the 
wing bud is just apparent to the naked eye as a small elevation on 
the body wall was chosen as the most advantageous for the opera- 
tion. The period of incubation before this stage is reached varies 
from 68 to 88 hours according to the time of year, the development 
of the embryo when the egg is laid, and fluctuations in the tempera- 
ture of the incubator, so that it is impossible to tell in exactly what 
condition the embryo will be found when the egg is opened, but 
Fig. 1, a photograph taken in this case after 74 hours of incubation, 
gives approximately the external appearance at the time when 
most of these operations were performed. ‘The wing, was never 
larger than this, and in the majority of cases it was smaller. Fig. 
2, a photograph of a cross-section in the region of the wing, shows 
the relation and development of different parts at this time. Even 
if the wing bud is removed close to the body, and no part of the 
wing proper develops, some of the muscles, or portions of them, 
extending dorsally from it to the vertebrz, and ventrally to the 
sternum, always appear. It, therefore, seems evident that the pri- 
mordium for these has not yet grown into the wing, but is still con- 
tained in the muscle plate. Parts of the bones of the shoulder 
girdle will also form, so that only a portion of their primordium can 
be located in the wing bud at this period. 
Lillie has determined that the wing develops from the 17th, 
18th, and 19th somites, and that it, with the other structures 
arising from these somites, is innervated by the 14th, 15th, and 
16th nerves, the sensory root arising from the 12th, 13th and 14th 
ganglia. Later in development these three nerve trunks unite to 
form a plexus before their distribution to the muscles and sensory 
areas of the arm, but at this stage they have not yet reached the 
place of union and each appears as a distinct trunk extending 
toward the base of the wing but not yet reaching it. Neither is it 
in contact with the myotome. 
