306 PROGRESS OF SCIENCE IN THE CENTURY. 



Waldejer stated in 1891 : — ^' A nerve fibre is an es- 

 sential part of the cell with which it is continuous 

 and the cell, its processes, the nerve fibre and the 

 collaterals which arise from the nerve fibre collect- 

 ively form a neuron or structural nerve-unit.'' * 



The kernel of the neuron-theory is in the con- 

 clusion that nerve-cell and nerve-fibre represent a 

 single cell, that the axis-cylinder of the nerve-fibre, 

 with its collateral branches and terminal ramifica- 

 tions, is, like a dendritic process, an outgrowth from 

 the nerve-cell. Verworn speaks of the triple founda- 

 tion of this doctrine, — (1) anatomical, (2) em- 

 bryological, and (3) experimental. 



(1) Kemak in 1838 and Ilelmholtz in 1842 had 

 shown the continuity of nerve-cell and nerve-fibre; 

 Deiters distinguished the axis-cylinder process from 

 the protoplasmic processes ; the methods of Golgi and 

 Ramon y Cajal, of Ehrlich and ISTissl, helped the his- 

 tologist to find his way in the maze ; the work of Kol- 

 liker, Waldeyer, Retzius, Lenhossek, Van Gehuchten, 

 Biedermann and many more gradually led the ma- 

 jority to the idea of the unity of the neuron. 



(2) To Prof. Wilhelm His in particular we owe 

 our knowledge of the development of a mother-nerve- 

 cell into a neuroblast and of this into a nerve-cell, 

 Avith a nerve-fibre, and dendrites. There is an unfor- 

 getable figure by Ramon y Cajal, which shows on the 

 upper line the increasing complexity of a certain kind 

 of nerve-cells in the series — frog, lizard, rat, man; 

 while the lower line shows five stages in the individ- 

 ual development of a neuroblast ; the result showing 

 the general parallelism between individual growth 

 and racial progress. 



* Sir William Turner, Pres. Address, Rep. Brit. Ass. for 

 1900. 



