OUTGROWTH OF THE NERVE FIBER 
843 
Taken t ogether with recent histogenetic studies, the experiments 
show that two element ar}^ phenomena are involved in ner\^e 
development: (a) the formation of the primitive nerve fiber 
through extension of the neuroblastic protoplasm into a filament 
— protoplasmic movement ; (b) the formation of the neurofibrillae 
within this filament — tissue differentiation. 
It is through the former that the specific nerve paths of the 
body are first laid down. The further analysis of the influences 
which determine these paths can be made only through the study 
of the laws which govern protoplasmic movement. 
The energy of outgrowth is immanent in the nerve cell, and the 
initial direction of outgrowth is already determined within the cell 
before the outgrowth actually begins. The formation of the 
fiber is therefore an act of self differentiation within Roux's defi- 
nition. 
One of the necessary conditions of outgrowth is in all probability 
a medium which affords some solid support to the fibers. 
The configuration of the various organs of the embryo affords 
certain paths of predilection, such as small channels or grooves, 
in which nerve fibers are found to grow. These factors, to- 
gether with the predetermination of the initial directions of out- 
growth within the cell and the motive force of the neuroblastic 
protoplasm itself, will account for the main features in the topog- 
raphy of the peripheral nervous system. 
The first nerves which form are composed of few fibers and have 
relatively short distances to grow before establishing connection 
>vith their end organs. The long paths found in the adult are 
largely the result of subsequent stretching or interstitial expan- 
sion, which takes place as the various parts grow^ or shift apart. 
The fibers which develop later follow, in the main, the paths laid 
down by the pioneers. 
The mechanism by which the proper connection between nerve 
fiber and end organ is brought about is not revealed by the ex- 
periments, though a certain analogy with the penetration of the 
egg by the sperm is suggested. 
