Differentiation of Neuroblasts 45 
cleus is less than it is in other specimens where the peripheral areas 
destroyed were no greater, so that both degeneration and lack of 
development must doubtless be taken into account. 
3. After Four Days of Incubation 
Experiment 19. This egg was incubated 4 days before the 
wing was removed, and preserved one day later. Only a slight 
elevation was left in the region of the right wing, otherwise the 
embryo was normal. ‘There was no demonstrable effect on the 
nervous system except that the branching of the peripheral nerves 
was less extensive and in some sections the ventral horn was prob- 
ably slightly decreased, a condition similar to that found when 
3-day-old embryos were operated on. ‘There was no degeneration 
of fibers or cells, although the ends of the nerve trunks were 
probably sectioned by the operation. This condition I shall 
attempt to explain later. 
F. Removal of Somites 
I then decided to study the effects of operating at an earlier 
stage by removing somites. By choosinga period when the primor- 
dium for all the muscles of a given somite is still to be found in the 
muscle segment, and destroying this, all the motor organs normally 
innervated by a given nerve would be extirpated. (The only 
possible exception being areas reached by the sympathetic system.) 
As nearly as could be determined, this condition is found in the 
wing region of the chick from about the 45th to the 6oth hour of 
incubation. At this period there is no visible differentiation 
within the medullary tube, and no outgrowth of peripheral fibers. 
It would therefore be possible to study the differentiation of the 
motor nucleus and motor root from the very beginning of their 
formation in the entire absence of all muscles which they nor- 
mally innervate. 
There are many technical difficulties to be overcome in experi- 
ments of this kind, but with suitable apparatus, and some experi- 
ence, it was found possible to remove the primordium of all the 
musculature of a given somite without injury to the spinal cord. 
