OUTGROWTH OF THE XERVE FIBER 
833 
Analysis of the factors which produce the specific arrangement 
of the nerve paths 
In proceeding with the analysis of our problem, the first point 
to consider is the localization of the energy which produces the 
outgrowth. The experiments answer this clearly: the primary 
act of extension by which the protoplasm of the neuroblast is drawn 
out into a fiber, the primordium of the axone, is due to forces im- 
manent in the neuroblast itself at the time when outgrowth be- 
gins and probabh^ for a considerable time before. This conclusion 
is based upon the direct observation that the movement takes 
place in the protoplasm itself without the application of any ex- 
ternal physical force, and it is corroborated by the fact that out- 
growth occurs even when the normal surroundings are radically 
modified, as in the present experiments or as Lewis ('07) and 
myself ('06, '10) have previously shown. That the original di- 
rection taken by the outgrowing fiber is already determined for 
each cell before the outgrowth actually begins, so that when it 
does begin it is dependent upon forces acting from within, fol- 
lows first from the fact that the nerve fibers within the embryo 
tend to grow out in a given direction even when quite different 
surroundings are substituted for the normal, and secondly from 
the fact that the nerve fibers which grow into the clotted lymph, 
are there surrounded on all sides by an isotropic medium, which 
cannot conceivably be held to produce movement in a definite 
direction. 
The formation of the protoplasmic nerve tracts falls, according 
to the foregoing, within Roux's definition of self-differentiation, 
by which is meant, not that the process is entirely independent of 
external conditions, but simply, as Roux ('85) in first defining 
the concept pointed out, that the changes in the system, or at 
least the specific nature of the change, are determined b}^ the 
energy of the system itself. In the particular case in question 
this means that within a medium compatible with the life and 
growth of the neuroblasts the formation of nerve fibers may take 
1^ Op cit., p. 423. In the " Gesammelte Abhandlungen," p. 15. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL, 9, NO. 4. 
