Differentiation of Neuroblasts 31 
has attained considerable size, but for the earlier stages, at which it 
was finally decided to operate, destruction by electrolysis is much 
more satisfactory. Number 12 sewing needles, one of them bent 
so as to penetrate the amnion and lie along the whole extent of the 
wing bud with as little tearing of the membrane as possible, were 
used for the operation. ‘These were held in electric handles con- 
nected with dry battery cells furnishing a current of about two 
volts. [he destruction is almost instantaneous, so that the current 
need be applied for but a very brief period. 
There existed the possibility that by this method of operation the 
electric current might penetrate beyond the region with which the 
needle was in contact, and thus lead to the direct destruction of 
cells within the spinal cord. Such a possibility is eliminated by 
the following facts:—in a number of embryos in which the wing 
was removed by scalpels the same defects appear as in those of the 
same age operated on by electricity. Moreover the defects always 
appear in the same region, and follow the same order, and it would 
scarcely be possible to suppose that an electric current, free to 
extend in any direction, should always follow exactly the same 
path and destroy the same cells, leaving those immediately sur- 
rounding them quite normal. And a third reason is found in the 
fact that until twenty-four hours after the operation the path of 
the current is well defined by means of the presence of the injured 
tissue, and in specimens killed at this period, and in which it is 
possible to determine by this means that the injury did not extend 
into or even close to the spinal cord, the nervous system is still 
defective and the defects are always of the same nature. 
I had at first many infected eggs, and occasional ones later, 
but as I have at other times operated every day for months with 
only one or two cases of infection, I believe that only sufficient 
care is necessary to avoid it entirely. ‘The surface of the egg was 
disinfected with mercuric chloride, and all instruments sterilized. 
There continued, however, to be a large percentage of loss from 
tears in the blastoderm, puncturing of bloodvessels, extensive 
injury to the amnion, and other causes whose nature was not 
determined. Probably most cases of death within a short time 
after the operation are traceable to injury to the circulatory sys- 
