OUTGROWTH OF THE NERVE FIBER 
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factors which influence the laying down of the specific nerve 
paths of any organism, we are concerned, therefore, primarily 
with the laws which govern the direction and intensity of proto- 
plasmic movement, and it is the analysis of these phenomena 
to which students of the ontogenetic and regenerative develop- 
ment of the nervous system must now direct their attention. 
The present discussion will not have been in vain if it makes 
clear that the development of the nervous system in the light of 
the protoplasmic movement concept is no less capable of rational 
analysis than is development in general. 
SUMMARY 
Reference is made throughout the following exclusively to the 
anouran embryo. 
Before histological differentiation of the medullary tube begins, 
its walls do not constitute a syncytium, but are composed of 
separate cells each with a distinct cell membrane, as freshly 
teased preparations show. 
The peripheral nerve fibers in their earliest stages, as seen in sec- 
tions of normal embryos, extend from the neural tube or cranial 
ganglia as finely branched processes of single cells, which in slightly 
later stages become extended to long fibers; the end of each fiber 
is a rhizopod-like structure with very fine processes or pseudo- 
podia. 
Pieces of undifferentiated embryonic tissue, when isolated under 
aseptic precautions in clotted lymph, will live for weeks and under- 
go at least the initial stages of normal histological differentiation : 
cells from the axial mesoderm give rise to striated muscle fibers; 
epidermal cells form a cuticular border; typical chromatophores 
and a mesenchyme-like tissue are formed from pieces containing 
portions of the neural tube and axial mesoderm; the walls of the 
neural tube and the primordia of the cranial ganglia give rise to 
long hyaline filaments closely resembling embryonic nerve fibers. 
Tissues grown in lymph function characteristically, as is seen 
in the movement of cilia and in the contraction of muscle fibers 
when left in organic continuity with fragments of the neural tube. 
