OUTGROWTH OF THE NERVE FIBER 
789 
monizing the two views. The wide discussion of the subject 
which has taken place reached a certain culmination in the contro- 
versy^ between Held and Ramon y Cajal in the years 1906-1909, 
in which it became clear that the evidence for and against the 
two theories respectively, rested upon such minute histological 
details that a decision to which all would subscribe was impos- 
sible of attainment. These two observers studied to a great 
extent similar material, often by the same methods, and, in fact, 
their prepared material was so much alike that Ramon y Cajal, 
after seeing Held's specimens, expressed great astonishment at 
the similarity." Yet the respective interpretations given by them 
differ diametrically. 
Under such conditions a search for evidence of other kinds is 
indicated. It was with the hope that a study of the problem by 
entirely different methods might yield such evidence, that the 
work described in the present paper was undertaken. A crucial 
experiment was sought that would decide between the two theo- 
ries. That a decision of this question is of fundamental impor- 
tance becomes apparent when we consider that the analysis of the 
factors bearing upon the development of this most intricate 
system of organs is wholly dependent upon it ; for it is obviously 
impossible to study intelligently the mechanics of development 
of the nerve paths, unless we know whether we are dealing pri- 
marily with phenomena of protoplasmic movement or with mere 
progressive differentiation without movement. 
An extensive series of experiments, as well as observations upon 
normal embryos, had led me previously to the adoption of the view 
of His and Ramon y Cajal. These experiments (Harrison '06- 
'10) showed that the ganglion cells within the nerve centers are 
the one essential element in the formation of the nerve fiber, in- 
asmuch as pieces of the embryonic nervous system transplanted 
to any part of the body may give rise to nerve fibers, while no 
fibers ever develop in the absence of ganglion cells. It was recog- 
5 R. y Cajal, 1908, p. 3, footnote : Tout recemment pendant un voyage en Alle- 
magne, nous avons eu le plaisir d'examiner k Leipzig, les excellentes preparations 
de M. Held. Ainsi que nous I'attendions elles sont tres reussies, mais k notre 
grande surprise elles montrent k peu pres les mdmes images que les notres. 
