OUTGROWTH OF THE NERVE FIBER 
827 
The latter is probably the result of neurofibrillation, since, as the 
work of Held and of Ramon y Cajal shows, the neurofibrillae 
extend almost to the tip of the fiber even in very young nerves. 
The movement of the neuroblastic protoplasm, which is brought 
about not by passive extension but by its own activity, will take 
place in a medium foreign to the embryonic body, and there can 
here be no possibility either of accretion by transformation of liv- 
ing protoplasm already in situ, or of outgrowth of fibrillar sub- 
stance within such protoplasmic connections, since there is nothing 
of the kind present, the solid parts of the culture medium being 
nothing but fibrin. Quite aside from this consideration, the char- 
acter of the movement, as observed directly, precludes all pos- 
sibility of extension according to the conception of either Hensen 
or Held. 
The criticisms of this conclusion that have appeared up to the 
present time, have been directed against my preUminary notices, 
in which the data were very briefly recorded and illustrations were 
few. Those who have expressed themselves adversely to the 
claim of conclusiveness are Hensen ('08), Schaeppi ('09), Kerr 
('10) and Held ('09). Hensen, however, raises no specific ob- 
jection and Schaeppi^^ merely not convinced that the actual 
growing end of the nerve fibers was observed. The criticism of 
Kerr ('10) is more specific, though of the same kind as that of 
^2 Schaeppi's criticism is directed mainly against my earlier experiments (Har- 
rison '06), which had not then been published in full. I regret that through in- 
advertence no notice was taken of Schaeppi's appreciative though adverse critique 
in my full paper (Harrison '10), in which the logical bearing of the various experi- 
ments is set forth at some length. No claim of absolute rigorousness of proof for 
the non-participation of protoplasmic bridges in the formation of nerves, was there 
made for any particular experiment, except in the case of the one in which the 
nerves grew within the implanted blood clot; and even in this case it was admitted 
as a remote possibility that the embryonic cells which rapidly organize the clot 
might form protoplasmic bridges. Taken together, however, these experiments 
afford a mass of evidence against the protoplasmic bridge theory, which to my 
mind far outweighs that which has been brought forward in its favor. The ex- 
periments certainly rob the theory of any claim upon functional activity as a fac- 
tor in the early development of nerves, even though the experiments with acetone- 
chloroform are not admitted as evidence. Turning now to the criticism of the 
present experiments, I feel confident that if Schaeppi had had before him the fig- 
ures which I am able to present here, he would hardly have asked: ''Wer in aller 
