14 Journal of Comparative Neurology. 



motor apparatus in the cord and its relations with muscles, etc., 

 external to the neuraxis. It may be assumed that the principle 

 enunciated by Hensen will be found to lie implicate in earlier 

 conditions than those which now prevail. 



It is intended to give at this time a part of the evidence 

 which led the writer to say in a previous number of this Journal 

 (p. cxxxix), " study of the growth of nerves in embryos of ser- 

 pents, amphibians and mammals has convinced the writer that, 

 in some cases at least, the growth is by moniliform adhesion of 

 neurons." 



The facts and observations will be given as briefly as possi- 

 ble without attempt to cite literature. Our thesis is the follow- 

 ing : Medullated nerve fibres are formed from proliferating cell- 

 masses which arise either within the neuraxis from niduli of mul- 

 tiplying cells (centrifugal fibres) or from spinal ganglia (centripe- 

 tal fibres.) The nerve grows by intrinsic proliferation due to 

 karyokinetic multiplication of the neurons. Such proliferation 

 may take place anywhere in the nerve. The nuclei of the neu- 

 rons at first lie in the fibre thus produced and only subsequently 

 are "side-tracked" and are connected with the sheath. They 

 probably retain a vital connection with the fibre. The sheath is 

 to be regarded as a peculiarly modified cell-wall of the neuron. 

 (It is possible that the segment-forming cells of the nerve-fibre 

 should be compared with spongioblast elements, and it is doubt- 

 ful whether an arbitrary separation can at first be made between 

 spongioblasts and neuroblasts, but the point here made is that 

 they are derivations of the neuraxis.) 



I. The nuclei of the nerve fibres are derived from the 

 neuraxis. Fig. i, of Plate II, illustrates the origin of the ocu- 

 lomotor nerve in a young snake embryo. In this case, as in all 

 other motor roots at an early stage, there is a peripheral migra- 

 tion of cells connected with the root. Other neuroblasts are 

 crowding peripherad from within. Some may be traced to the 

 boundary of the medulla and resemble in every respect others 

 which lie in the peripheral part of the nerve. Throughout the 

 entire course of the nerve it is made up of neurons of a similar 

 sort. Fig. 3, of the same plate indicates the relations in a sala- 



