seu M. L. Shorey 
thus be accounted for, and it cannot be asserted that the condition 
found in the older embryos is due to degeneration rather than to 
lack of development, unless there is evidence of degeneration, or 
it can be shown that the differentiation of the motor horn is once 
completed, and the motor roots at some period contain the full 
number of fibers, for the question of self-differentiation must be 
reduced to a consideration of individual cells, and under the con- 
ditions of Braus’s experiments any neuroblast which normally 
differentiates into a motor nerve, and failed to do so here, could 
not be self-differentiating. 
B- Purpose, Methods, and Material 
The following experiments were conducted for the purpose of 
making further observations on this point: 
Obviously the method of studying this problem by extirpating 
limb buds cannot give crucial results with any animal in which 
the regeneration of these parts occur, and I have not succeeded in 
getting amphibian material in which the limbs are not regenerated 
repeatedly. In my experiments I found that Bufo americanus 
regenerated less rapidly, and perhaps less completely, than Ambly- 
stoma tigrinum or Rana pipiens, and probably in no instance were 
all vestiges of musculature connected with the limb removed. 
Whatever the cause, I have no specimen which was allowed to live 
long enough for regeneration to be expected in which it did not 
occur. Nevertheless I believe that the results obtained indicate 
that the process of differentiation of the neuroblasts is essentially 
the same in the amphibians as it is in the chick. 
Amblystoma tigrinum, Rana pipiens, and Bufo americanus were 
used. With Amblystoma, sometimes the fore limb, sometimes the 
hind limb, and sometimes both were removed, either when the 
limb bud was just apparent or before it had attained any consider- 
able size, and in many instances the operation was repeated as 
often as the beginning of the formation of a new limb bud could be 
detected. ‘The limbs develop slowly, and in the early stages a 
large proportion of the musculature innervated by the sciatic plexus 
forms such an intrinsic part of the body of the tadpole that the 
removal of the projecting portion of the limb leaves many muscles 
