840 
ROSS GRANVILLE HARRISON 
be necessary to account for the definitive establishment of partic- 
ular nervous connections. The few experiments which I have 
directed to this end have given negative results, which is not 
surprising when the crudities of the method are borne in mind, 
but since it is possible to introduce many refinements into these 
methods, an ultimate solution of the problem in this way does not 
seem to be beyond hope of attainment. 
The specific arrangement of the fibers within the central nervous 
system affords a morphogenetic problem of much greater difficulty. 
Still there is nothing in the conditions in the walls of the neural 
tube which is inconsistent with the development of the nerve 
fibers in accordance with the view here represented. The grow- 
ing fibers are clearly endowed with considerable energy and have 
the power to make their way through the solid or semi-solid pro- 
toplasm of the cells of the neural tube. But w^e are at present 
in the dark with regard to the conditions which guide them to 
specific points. 
In pointing out the above factors which seem to be involved 
in the development of the nervous system, I am aware of the great 
imperfections in our knowledge of the subject, and of the little 
progress that has been made beyond the ideas of His and Ramon 
y Cajal. Nevertheless, although our present conception of the 
secondary factors which influence the nerve paths may have to be 
modified in the light of future knowledge, the primary factor, 
protoplasmic movement, must be regarded as definitely estab- 
lished and it will have to form the basis of any adequate theory 
of nerve development. The chief claim to progress that the pres- 
ent work has is that it has taken this factor out of the realm of 
inference and placed it upon the secure foundation of direct 
observation. With this it has been shown that the first man- 
ifestations of activity observable in the differentiating nerve cell 
are of the same fundamental nature as those found not only 
in other embryonic cells but also in the protoplasm of the widest 
variety of organisms. The movement which results in the drawing 
out of a compact cell into a long filament, the primitive nerve 
fiber,- it is but a specific form of that general type of movement 
common to all primitive protoplasm. In studying the secondary 
