OUTGROWTH OF THE NERVE FIBER 
835 
('98 and '07) has thought to be the case in the regenerative 
growth of epithehum. This, however, would not necessarily 
preclude the possibility of the bridging of very small spaces by 
the outgrowing fiber. 
There are cases which have been described where nerve fibers 
are found extending for considerable distances in the ventricular 
fluid, as for instance Reissner's fiber, and all those cases which 
Held refers to as developmental curiosities, but these do not 
contradict the view here advanced, for it is quite possible that 
the fibers originally grew along the surface of the lining of the 
neural tube. I am also of the opinion that the case described by 
me in which a nerve was found crossing the peritoneal cavity is 
not altogether against this view, because the attachment of the 
nerve was probably effected before the separation of the splanch- 
nopleure and somatopleure took place. 
Given a form of protolasm with power to extend itself in a defi- 
nite direction so as to form a fiber, the next step is to deter- 
mine the influences which may modify the direction of its growth 
and produce the specific arrangement of nerve tracts found in the 
mature organism. His ('88) was the first to show that develop- 
ing nerves in the normal embryo begin their growth in a straight 
line, and that this direction may afterward be modified by various 
agencies such as the shifting of parts, or by meeting obstacles in 
the path of grow^th. The normal amphibian embryo affords 
abundant confirmation of these observations which His made 
upon the human embryo. The peripheral fibers from the dorsal 
nerves of Rohon-Beard, for instance, run at first laterally in a 
straight line through the notches between the muscle plates ; they 
soon reach the epidermis and are there deflected through an arc 
of nearly 90°, to a dorso-ventral direction, with other minor de- 
flections of a variable nature. In many cases it is apparent that 
the nerves follow definite paths, which are preformed in the sense 
first nerves to grow did so independently of the peripheral conditions, as my own 
experiments show. The inhibition has affected principally the fibers which grow 
later, when distances are much greater and conditions much more complex; in 
other words, it has affected those fibers which grow in the period when differentia- 
tion is more dependent upon function, rather than those of the earlier period of 
self-differentiation. 
13 Cf. Ramon y Cajal, 1908. 
