126 PROFESSOR J. GRAHAM KERR ON SOME POINTS IN THE 
Lepidosiren offers no evidence, so far, as to the ultimate origin of the nerve fibrils. 
They appear gradually in an at first simple protoplasmic matrix. ‘hey seem to form, 
as it were, a kind of picture in morphological symbols of already existent physiological 
activities. It would seem that the continued passage backwards and forwards between 
centre and end organ of a stream of nerve impulses gradually finds expression in the 
marking out of the original simple-looking granular protoplasm into definite fibrillar 
impulse tracks,* the undifferentiated protoplasm remaining as the _perifibrillar 
substance. Such a view of the nature of the fibrils is supported by Berue’s remarkable 
observation t that in the chick embryo about the end of the sixth day a nerve trunk 
may lose its fibrillar character in the immediate neighbourhood of a mitotic figure, to 
reassume it on the completion of mitosis. 
It is pretty clear that the great function of the sheath is to serve as a nutritive 
organ for the nerve trunk. We see how its protoplasm is at first laden with yolk which 
gradually becomes used up as the nerve trunk develops within it. 
That the main function of nuclei, apart from reproduction, is to control cytoplasmic 
metabolism is well recognised. The nuclei of the sheath are able to exercise this 
control over the active metabolism of the developing nerve trunk which is without nuclei 
of its own. Connected with this relation of the sheath nuclei to the metabolism of 
the nerve trunk is no doubt the active multiplication of these nuclei observed in 
early stages of nerve regeneration.{ In such regeneration it may well be that the 
protoplasmic matrix of the nerve simply repeats the process of its original develop- 
ment, increasing in size and then developing nerve fibrils within itself. If these 
fibrils represent merely the differentiated paths of nerve impulses passing through 
the substance of the protoplasm, it would of course: happen naturally that the 
regenerated fibres would be formed in continuity with those of the undegenerated 
stumps. On this view the process which takes place when the peripheral 
part of a cut nerve degenerates and then regenerates is somewhat as follows :—The 
fibrils, no longer subject to the stimulus of passing nerve impulses, revert to their proto- 
plasmic condition. The protoplasmic sheath becomes highly active.§ It increases in 
thickness: its nuclei divide actively. Its protoplasm digests the remains of the 
medullary sheath.|| It thus comes to contain stored-up food material as in its original 
embryonic heavily yolked condition. The protoplasmic matrix representing the 
degenerated axis cylinder lies imbedded within the sheath.{1 Controlled and supplied 
with nourishment by the activities of the surrounding sheath the protoplasm behaves 
just as it does in ontogenetic development: (1) it grows—probably slowly—and 
* Were this the case, it might well be that the formation of fibrils might tend as a rule to spread from the end of 
the nerve from which came the most active and frequent nerve impulses. 
+ Berun, Allgemeine Anatomie wnd Physiologie des Nervensystems, p. 244. 
t Bonener, Ziegler’s Beitrdge z. Path. Anat., Bd. x., 1891. 
§ WrErING, op. cit., Bd. xxiii., 1898. 
|| This view of nerve regeneration, which my ontogenetic work inclines me towards, appears to agree most closely 
with that enunciated by Neumann (Arch. Path, Anat. u. Phys., Bd. clviii. p. 466). 
‘1 This protoplasmic strand within the protoplasmic sheath could only be demonstrated with extreme difficulty. 
