MULTIPLE NEUROMATA OF THE CENTRAL NERVOUS SYSTEM. (Ewe 
ferent extra-ganglionic substance. In order to confirm this observation Harrison 
carried out a further series of experiments. Bravus and Biancut had previously trans- 
planted buds of larval extremities, in which there were no nerves at the time of trans- 
plantation, and had found nerves developed autochthonously with no connection with the 
nerves of the host. Similar experiments carried out by Harrison and Lewis led them 
to the conclusion that nerves are not formed i situ in the transplanted limbs but grow 
into them from the nerves of the host, and that there is no evidence that any specifi- 
eally formed or localised structures, essential to the formation of nerve fibres, are present. 
Harrison's final step was to answer the question: Is the nerve fibre entirely the 
product of the nerve centre? He recognised that in all his former experiments the 
nerve fibre had developed in surroundings composed of living organised tissue which 
might possibly contribute organised material to the nerve elements. He initiated, there- 
fore, what he describes as a really crucial experiment. ‘This consisted in the placing of 
pieces of embryonic tissue, taken before any histological differentiation has taken place, 
in hanging drops of clotted frog’s lymph, and keeping the sealed preparations under 
observation for a number of days. ‘The cells when taken were rounded, without any 
sign of differentiation, and were found soon to manifest amceboid movement—resulting 
in the formation of long threads of hyaline protoplasm with free filaments which con- 
tinually change their form and are exactly similar to the pictures by Casa in normal 
embryos. It is to be noted that cilia of neighbouring epidermic cells remained active 
and embryonic mesoblast cells became transformed into striated muscle fibres, so that 
there was no doubt that even under artificial conditions life and growth and differentia- 
tion were continuing. The development of the nerve fibre is thus brought about by 
one of the primary properties of living protoplasm common to all cells—amoeboid move- 
ment, and Harrison points out that he had substituted for the supposedly-essential 
protoplasmic bridges only unorganised fibrin threads which could afford merely a 
mechanical support for the growing nerves. The elementary factors in nerve develop- 
ment are therefore two—the one, protoplasmic movement, the other, the differentiation 
of this protoplasm by the formation within it of neuro-fibrils. 
Heip and Harrison differ as to the source of the protoplasm within which the 
neuro-fibrils develop. HEtp believes that it is formed of cells scattered all through the 
embryonic body : Harrison that it flows out from the central cells and thereby estab- 
lishes the path in which the necessary fibrils are formed. It is this laying down of the 
path by means of a form of protoplasmic movement, rather than the process of differen- 
tiation into neuro-fibrils, that constitutes the problem in the development of nerves. 
CarPENTER and Marn (1907), in pig embryos, have traced cells which migrate from 
the medullary tube, pass into the ventral nerve roots, and form the sheath cells. Kuntz 
(1909), also in pig embryos, has made similar observations in relation to both ventral 
and dorsal roots. He states further that these migrated cells pass along the spinal 
nerves and ventral rami to form the anlage of the sympathetic ganglia. All these 
writers refer to the cells as the “ indifferent” cells of ScHAPER. 
