826 
ROSS GRANVILLE HARRISON 
and degree of their motility, the quality of their differentiation, 
and their mode of physiological activity when differentiated. 
Even had we but scant knowledge of the normal development 
of nerve fibers, we should therefore be justified in concluding that 
the phenomena witnessed in preparations from the central nervous 
system are a representation of what occurs when the nerve fibers 
develop in the body of the embryo. The study of normal develop- 
ment strengthens this conclusion immeasurably by revealing, in 
the young stages of peripheral nerves, structures which are strik- 
ingly similar to the protoplasmic fibers found in the lymph. 
(Cf. figs 4-6 with figs. 8, 9 and 21). Such fibers with their 
active amoeboid ends are formed by cells taken both from the 
walls of the medullary tube and from the rudiments of the cran- 
ial ganglia, and must be regarded as specifically nervous. It is 
thus scarcely conceivable that the identity between the fibers 
found in the lymph and true embryonic nerves can be questioned, 
for that conclusion, as just pointed out, is based not only upon the 
general fact that the differentiations taking place in the isolated 
tissues are normal and specific, as far as they go, but also upon the 
particular morphological resemblance between the structures in 
question and undoubted nerves. It must be admitted that the 
case might be made even stronger were it possible to preserve the 
isolated nerves satisfactorily and stain them by the specific stains 
for neurofibrillae, though this is in no sense essential to the argu- 
ment here advanced (see footnote, p. 807). 
The hearing of the experiments upon the theories of nerve 
development 
As the experiments clearly show, one of the fundamental char- 
acteristics of the neuroblastic protolplasm is its high degree of 
motility, which, being manifested by only a limited portion of the 
cell, results in the drawing out of the protoplasm into a long fila- 
ment representing the axone of a nerve fiber. The extreme tip 
of the fiber, which is the cone accroissement of Ramon y Cajal, 
remains remarkably mobile, while the body of the fiber evidently 
soon acquires a firmer consistency and considerable tensile strength. 
