832 
ROSS GRANVILLE HARRISON 
analogy between his view and the one advocated liere. It is, how- 
ever, the peculiar relation of the outgrowing nerve substance to 
the intercellular net work to which Held attaches the most funda- 
mental importance, and it is in this respect, as comparison of his 
work with, that of Ramon y Cajal shows, that he goes further than 
the methods he has employed w^ould, in my opinion, warrant. 
What Held's figures really show, objectively expressed, is that the 
neurofibrillae are laid down within protoplasm and that they are 
always connected with the neuroblast, extending further peripher- 
ally in later than in earlier stages of development. This is in no 
wise incompatible with the results of my experiments, for the only 
difference concerns the source of the protoplasm in which the fibril- 
lae develop, Held holding that it is formed of ceils scattered all 
through the embryonic body, while I maintain on the basis of the 
experiments, that it flows out from the central cells, and thereby 
establishes the paths in which the neurofibrillae are formed. 
But it is this laying down of the primary nerve paths by means 
of a form of protoplasmic movement, rather than the process of 
neurofibrillation, that constitutes the specificalh^ intricate problem 
in the development of the nervous system. The differentiation 
phenomena are naturally of great interest too, but though chem- 
ically specific, they are essentially of the same class as the differ- 
entiation phenomena witnessed, for example, in muscle or in the 
connective tissue cell ; and they are not in any way comparable, 
as regards complexity of special relations, to the phenomena of 
the establishment of the primitive nervous connections by the 
outflow of the neural protoplasm. In order to discover the 
factors which influence the formation of the nerve paths, we 
must, therefore, in the first instance take into consideration this 
property of protoplasmic movement. This is of the utmost 
importance, and any theory of nerve development which fails 
to do so is sure to be misleading. 
