OUTGROWTH OF THE NERVE FIBER 
839 
takes place during development after the first connections between 
the nerve fibers and the epidermal cells have been established. 
There is nothing in the present work which throws any light 
upon the process by which the final connection between the nerve 
fiber and its end organ is established. That it must be a sort of 
specific reaction between each kind of nerve fiber and the partic- 
ular structure to be innervated seems clear from the fact that 
sensor}^ and motor fibers, though running close together in the 
same bundle, nevertheless form proper peripheral connections, the 
one with the epidermis and the other with the muscle. That the 
connection is not long deferred is shown in a large number of 
instances where the nerves reach their end structures, and func- 
tion is established very early in development. The foregoing 
facts suggest that there may be a certain analogy here with the 
union of egg and sperm cell. The nerve fiber during its growth 
comes into contact with a cell of the proper kind. Assuming the 
latter to be in a condition of ripeness, a more intimate contact or 
perhaps even actual fusion may take place between nerve twig 
and end cell. A connection of this kind once established would 
terminate the susceptibility of the cell to further innervation, 
and nerve fibers growing subsequently in the same path would 
pass along to other end cells which were mature but not yet in- 
nervated. The nerve fiber itself, however, apparently retains its 
power of grow^th and ramification, for it usually becomes con- 
nected finally with a large number of end cells, as is plainly the 
case with muscle and ordinary cutaneous endings. It is in the 
establishment of the definitive connection with end organ rather 
than in the determination of the direction taken by the main 
nerve trunks, that influences such as chemotaxis may be expected 
to operate. 
The present experiments suggest, of course, a possible method 
for further study of this problem. If it could be shown that 
there is an attraction between growing nerve fibers taken from 
a certain part of the nervous system and a particular kind of 
peripheral cell, and between another type of central neuroblast 
and a different peripheral cell, then we should have direct evidence 
for the existence of those more subtile factors which seem to 
