Ross Granville Harrison 129 
This was done by removing the medullary cord of the trunk shortly after 
its closure. The result is always the total absence of peripheral nerves, 
except the cranial. In the second set of experiments the peripheral path 
was altered. The simplest way to accomplish this is to remove the spinal 
cord before any nerves are visible. After this the wound heals readily 
and during the next week at least no regeneration takes place. Above the 
notochord in the trunk of the embryo there is thus left a small space 
which becomes filled with mesenchyme. Into this the longitudinal bundle 
fibers arising in the brain grow, and after a few days they may be fol- 
lowed as far as six or eight segments from the cut end of the medullary 
tube. In other words, fibers which normally develop in the walls of the 
latter, develop here within the mesenchyme, which is a tissue as unlike 
that forming the normal path as it could possibly be. 
The third mode of experimentation, which is not formally different 
from the preceding, consisted in the transplantation of parts of the central 
organ. In one series of experiments the spinal cord of the embryo was ex- 
tirpated, and in each case a small piece of the cord was transplanted 
under the skin of the abdominal walls. The normal nerves of the body 
of course do not develop in such cases, but small nerve trunks do arise 
from the transplanted pieces and run for some distance in various direc- 
tions, usually remaining in the abdominal walls. Sometimes portions of 
the ganglion, crest were transplanted with the cord, resulting in the for- 
mation of small ganglia. In one of these instances, already referred to 
above, the nerve fibers, which were sheathless, ran free through the peri- 
toneal cavity. While the great length of this nerve is due largely no 
doubt to the shifting of its peripheral attachment, it is nevertheless quite 
impossible that preformed bridges could have been present in its course. 
The foregoing results can be interpreted in but one way. The nerve 
center (ganglion cells) is shown to be the one necessary factor in the 
formation of the peripheral nerve. When the former is removed from 
the body of the embryo the latter fails to develop. When it is transplanted 
to abnormal positions in the body of the embryo it then gives rise to 
nerves which may follow paths, where normally no nerves run, and like- 
wise when the tissues surrounding the center are changed entirely, nerves 
proceeding from that center may develop as normally. The nerve fiber 
is therefore a product of the ganglion cell. The histological findings in- 
dicate that it is an outflow of the substance of the ganglion cell and not 
a mere activation by contact of indifferent extra ganglionic substance. 
While Lewis’s * experiments upon the olfactory and optic nerves afford 
