464 Evidence of the Outgrowth of the Axis Cylinder 
the place of attachment of the eye was injured at the time of extirpation 
of the optic vesicle. These injuries have in some way resulted in 
irregularities in the brain in this region, that have given rise to extra 
nerves. ‘These nerves extend from a region of the brain which, under 
normal conditions, never gives rise to peripheral nerves, or at least to 
nerves that leave the brain in the region to run into the mesenchyme. 
These nerves are, of course, unilateral and follow strange paths in the 
mesenchyme. The injury in the brain in these experiments was done 
long before the nerves normally appear. The nerves were not dragged 
out from the brain by the needle. The injury in some way made a 
path perhaps and may even have altered the orientation of some of 
the nerve cells. 
In experiment bx, (Fig. 11) the fore part of the brain was removed 
before there were any traces of nerve fibers, without injury to the eyes 
or nasal pits. From the latter have arisen the olfactory nerves, which 
extend in various directions in the mesenchyme without reaching the 
brain. ‘The nerve fibers are evidently outgrowths of the cells in the 
olfactory organ and are in no way connected with a central organ. It 
would seem impossible to explain their presence by the Hensen doctrine. 
In like manner the nerve fibers arise from a transplanted nasal pit. 
Fig. 12 is from a nasal pit which was transplanted beneath the ectoderm 
dorsal to the eye. After transplantation differentiation of the organ 
continued and nerve fibers were sent off into the region between it and 
the ectoderm, Fig. 13, but extend only a short distance. 
The extraordinary interesting behavior of the optic nerve in the 
transplanted eyes afford very striking evidence of the outgrowth of the 
optic nerve from the eye. 
The optic vesicle in these experiments was cut off and transplanted 
into the otic region of the same embryo, shortly after closure of the 
neural folds and long before the optic nerve forms. 
In the majority of transplanted eyes the optic nerve pierces the retina 
as far as the outer or pigment layer, it then takes a course in among 
the pigment cells, often running for long distances as a compact bun- 
dle between these cells, and finally ending among them without enter- 
ing the mesenchyme. Such nerves do not seem to be provided with 
sheath cells. Occasionally the optic nerve passes through the pigment 
layer into the mesenchyme and after running in it a short distance ends 
there. 
In one embryo the optic nerve was evidently turned from its course 
in the outer molecular layer, in which layer it can be followed for many 
sections, and ends there without reaching even the pigment layer. Some- 
