830 
ROSS GRANVILLE HARRISON 
study of normal material, however clearly stained, that they are 
essential to nerve building. So long as we keep an animal in pure 
air without ever varying its surrounding medium, we have no 
means of knowing whether the nitrogen constituent is essential 
to its life or not. It is only by eliminating or at least by varying 
this part of medium that it can be shown not to be necessary. In 
the embryonic body, according to the descriptions of Held, no 
nerve can grow along a normal path without coming into inti- 
mate contact with the protoplasmic bridges or the protoplasm of 
the cells within the central nervous system. Until these are 
eliminated or modified, therefore, we can have no knowledge 
whether they are essential to the growth of the nerves or not. 
This is the crux of the whole question and it is this that the present 
experiments have settled, adversely to the view taken by Held, by 
substituting for the supposedly essential protoplasmic bridges, 
unorganized fibrin threads, which afford merely mechanical sup- 
port to the growing nerves. In view of this I find it altogether 
impossible to accept Held's conception as correct, though just to 
what extent the protoplasmic net work which he describes may 
influence mechanically the growing fibers remains problematical, 
there being no ground for denying it a place as a subsidiary 
factor along with the other structure^ of the embryonic body. 
Notwithstanding this difference of opinion I think that it will 
become clear from the following, that, in the main, the relation 
between Held's work and m}^ own is not antagonistic but comple- 
mentary. 
The elementary factors of nerve development 
When we consider the elementary phenomena of nerve develop- 
ment in the light of the present experiments and of recent histo- 
genetic studies, the first thing that stands out is that two separate 
processes are involved: the one is the protoplasmic movement, 
which results in the drawing out of a part of the neuroblast into 
a thread of protoplasm, the primitive nerve fiber ; the other is the 
difffirentiation of this protoplasm by the formation within it of 
neurofibrillar substance. These may be referred to as the motor 
and the diiferentiation phenomena respectively. The existence 
of the former has either not been recognized or has been openly 
