Differentiation of Neuroblasts 39 
develop more slowly, and the condition of the operated side is 
merely an earlier stage of development. 
2 The missing neive elements have formed, and then degener- 
ated. 
3 The neuroblasts are in some way dependent on the surround- 
ing tissue, particularly their normal end organs, for development, 
and when these are wanting they do not differentiate. 
The first hypothesis may be disposed of at once by comparing 
the defective cases with earlier stages of development (figured 
on the normal side of the experiments which follow), from which 
it will be seen that the condition in the operated embryos is not 
found at any time in the normal process of growth. Considering 
the stage of development that had been reached at the time of the 
operation, it is evident that if the second hypothesis is correct, the 
missing nerve elements must have both formed and degenerated 
within five days. And the degeneration must have been com- 
plete, for no cells in the process of destruction could be found. 
The rapidity of development in the chick makes this possible, 
perhaps, and the question could be decided only by an examina- 
tion of earlier stages. 
Specimens Preserved from Four Days to One Day After the 
Operation 
Three specimens were preserved on the fourth day after the 
operation. In one of these the injury was so slight that it was of 
practically no value. In another the spinal cord was apparently 
directly injured, and this was also discarded. In the third the 
operation and results were as follows: 
Experiment 179. The egg was incubated for 66 hours, and 
at the time of the operation the wing bud was scarcely distinguish- 
able. As a result of operating at this early stage it was found on 
examining the embryo 4 days 1 hour later that a small portion of 
the wing had not been removed, and that the body wall for a con- 
siderable distance posterior to the wing was entirely absent. 
Only the merest remnant of the scapula was left and this was 
much out of position; the coracoid was present but abnormal in 
position, shape and size. The humerus was also abnormal, and 
