54 M. L. Shorey 
medium surrounding the neuroblasts in its immediate neighbor- 
hood, and thus a change in the chemical inter-reactions may be 
effected. This would give at least a possible explanation of the 
influence of the muscles on the nerves by which they are normally 
innervated. Moreover, although the metabolic products of mus- 
cular activity would necessarily be found in greatest amount in the 
somite in which the muscles were located, with the free exchange 
of lymph they must also affect, to some extent, the character of the 
lymph in adjacent somites. This would give a perfectly natural 
explanation for the differentiation of some motor nerves in somites 
in which the primordium of all the muscles had been extirpated. 
And it is very significant that in such cases, when the fibers are 
sufficiently developed for connection with the end organs to be 
apparent, they are found to innervate structures in the somite 
above or below. I therefore regard the development of some 
motor nerves in regions where the muscles which they normally 
innervate have been destroyed, as in no way inconsistent with the 
belief that neuroblasts do not differentiate into motor nerves with- 
out the presence of muscular end organs, but rather as a neces- 
sary corollary of this condition. And as these experiments clearly 
demonstrate that the majority of neuroblasts which normally 
develop into motor nerves show no sign of differentiation in the 
absence of their end organs, I conclude that in the chick all such 
neuroblasts are entirely lacking in the power of self-differentiation. 
III EXPERIMENTS ON AMPHIBIANS 
A Historical and Critical Survey of Previous Work 
It isa universally accepted principle in biology that the funda- 
mental properties of protoplasm are essentially the same for all 
organisms, otherwise it is useless to attempt to formulate any 
general laws governing the animal world. And whether an organ 
or a cell attains its adult condition because of powers inherent 
within itself, or whether it must be acted on by outside forces to 
reach this condition, must be regarded asfundamental. Yet other 
investigators, working on different forms, have reached the conclu- 
sion that the neuroblasts are entirely self-differentiating, and one 
