Differentiation of Neuroblasts 27 
vated. A few experiments dealing with the effect of an abnormal 
environment on the developing nerve have, however, been made. 
Harrison (’06), experimenting on frog larve for the purpose of 
ascertaining whether other cells than those of the nerve center 
enter into the formation of the nerve fiber, obtained results which 
bear on the question under consideration. ‘The spinal cord of 
these embryos was extirpated, and small pieces of it transplanted 
under the skin of the abdominal walls. Small nerve trunks arise 
from these and run for some distance in various directions. From 
portions of the ganglionic crest thus transplanted small ganglia 
and nerves may arise. His own conclusions are as follows: 
“The nerve center (ganglion cells) is shown to be the one necessary 
factor in the formation of the peripheral-nerve. When it is trans- 
planted 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 likewise when the tissues surrounding the center 
are changed entirely nerves proceeding from that center may 
develop as normally.” In another experiment fragments of the 
medullary tube, isolated before visible differentiation, were placed 
in a drop of lymph, and it was found that these develop short 
fibers. From this experiment he concludes that the outgrowth of 
the fiber is largely independent of external stimuli, although its 
direction is doubtless influenced by a number of factors. ‘That 
the direction is influenced is shown by a number of experiments in 
which a limb bud transplanted to some other region of the body 
receives normal innervation from the nerves of the region to which 
it is transplanted. 
Braus (’06) removed one of the anterior limb buds of several 
toad larvez, and ten days later found that the brachial plexus was 
as well developed on the injured as on the uninjured side, although 
by far the greater part of the musculature had been destroyed, and 
at the time of the operation no nerve fibers running to the limbs 
were visible. Older operated embryos showed the brachial plexus 
present but diminished in size. From these experiments Braus 
concludes that “Der Befund am Plexus brachialis ist eine sehr 
deutliche Illustration dafiir, dass die Entwicklung der peripheren 
Nervenfasern unabhangig ist von den Endorganen.” 
