NEURONE THEORY 157 



fiber. The earlier relations are of course modified during growth; the 

 definitive relation between nerve and end-organ are acquired by mutual 

 adjustment. Recently Harrison has contributed further experimental -evi- 

 dence in support of the view that the growing axon is guided through a 

 stereotropic response (Jour. Exp. Zool., 17, 4, 1914). 



By cultivating sympathetic neurons from pieces of the intestine of the 

 embryo chick in saline solutions, W. H. and Margaret R. Lewis (Anat. Rec., 

 6, 1, 1912) have been able to demonstrate that here also the fibers arise as 

 outgrowths of nerve cells. 



NEURONE THEORY 



The work of Harrison, the Lewises and many others, including both 

 experimental and morphological investigations, leave scarcely any fur- 

 ther doubt of the accuracy of the Neurone Theory of Waldeyer (1891), 

 which simply applies the Cell Theory of Schleiden and Schwann (1838- 

 39) to the nervous system. It holds that the unit of structure is the 

 neuron (neurocyte), consisting of cell-body (cyton) and processes, in- 

 cluding one axon, and one or several dendrons. The nervous system 

 consists therefore of innumerable associated neurons. Neurons arise 

 each from a single embryonic cell, the neuroblast, retain their independ- 

 ence throughout life, and make connection with each other in general 

 only by contact, which, however, is sufficiently intimate to insure func- 

 tional continuity. A neuron exhibits a structural and functional po- 

 larity; and constitutes a trophic unit for the maintenance of whose 

 metabolic activity a nucleus is necessary. 



Further confirmation of the outgrowth interpretation as opposed to 

 that of autogenesis of the axon has recently been furnished by the experi- 

 ments of Clark (Jour. Comp. Neur., 24, 1, 1914) on the domestic fowl. 

 By prolonged exclusive feeding of polished rice he induced degeneration 

 in the peripheral medullated nerves. On return to an adequately nutritive 

 diet regeneration, accompanied by a return to normal locomotion and func- 

 tion, followed. The material thus gave opportunity for a microscopic study 

 of the steps in the nerve degeneration and regeneration. When the degen- 

 erative process had not been excessively prolonged a new axis cylinder grew 

 down the old medullary sheath, which returned to normal; when greatly 

 prolonged the myelin disappeared and the nuclei of the neurolemma multi- 

 plied, giving an appearance very similar to that of embryonic nerve fibers 

 ('bandfasern' stage). Clark concludes that the function of these excessive 

 sheath cells is the removal of the degenerating myelin, a new medullary 



