834 
ROSS GRANVILLE HARRISON 
place without the application of any further external force either 
as stimulus or as motive power. 
The experiments indicate that some solid support is one of the 
essential conditions of growth, the fibrin threads apparently 
affording this support in the experiments with clotted blood and 
lymph. At least it has not been possible to induce growth in 
purely fluid rnedia, and I am therefore inclined to hold to the hy- 
pothesis stated in the beginning (see p. 800), that some form of 
stereotropism plays a role in the outgrowth of the fibers, as Loeb 
Miss Shorey ('09) objects to calling this self-differentiation on the ground 
that the lymph used in the experiments contains the products of metabolism of 
various organs of the body, including organs such as muscles whose physiological 
activities are similar to those of the embryonic parts which the nerves in question 
would normally innervate; and these products of metabolism are the substances 
which, according to Miss Shorey, stimulate the growth of embryonic nerves . While 
it is of course true that lymph is a complex medium and may contain a variety of 
such products, it is nevertheless pretty hazardous, to say the least, to assume that 
they are the same in the lymph of an adult frog as in the interstices of a young em- 
bryo, and, in the absence of experimental evidence, it is entirely without foundation 
to assume that it is these particular products that stimulate the neuroblasts to grow 
out. In my opinion, the lymph is merely a medium in which these structures are 
capable of growing as a mouse grows in air or a fish in water. There is no specific 
relation between these media and the kind of organism that develops. Roux had 
such cases in mind in casting his definition of self-differentiation. Within the 
lymph the growing nerve fiber is bathed on all sides by the same medium and it 
is impossible therefore for the latter to exert any directive action. In interpret- 
ing her own striking and very important experiments Miss Shorey has ignored a 
number of facts, brought out by Lewis ('06 and '07) and myself (Harrison '06), 
which are opposed to her theory, as, for instance, the experiments in which part of 
the medullary tube was removed, with the result that the longitudinal fibers from 
the remaining part grew straight out into the mesenchmye. Since these fibers 
would normally have grown within the substance of the cord, and since, to carry 
out Miss Shorey's hypothesis to its logical conclusion, it must be supposed that 
that tissue emits some product of metabolism capable of stimulating the growth 
of the intrinsic nerves, we should consequently find in the experiments the self 
contradictory condition of nerve fibers growing directly away from that particu- 
lar tissue, the remaining part of the neural tube, which alone should be stimulating 
growth toward it. 
Miss Shorey's experiments show that the extirpation of peripheral material has 
a marked effect in partially inhibiting the development both of the nerves supply- 
ing it and of the central neuroblasts themselves. It is significant that in these 
experiments the effect of removal was found to be much more marked after a con- 
siderable time had elapsed, and also that in no case was the development of pe- 
ripheral nerves and their neuroblasts entirely suppressed. This indicates that the 
