390 THE WONDER OF LIFE 



growth of nerve-fibres and the estabhshment of specific 

 nervoiis connections on which the efiectiveness of sub- 

 sequent activity depends. In 1890 Ramon y Cajal dis- 

 covered at the end of the embryonic nerve fibres, at a very 

 early stage in their development, what he called a cone 

 of growth, which he compared to the finger-like outgrowth 

 or pseudopodium which the Amoeba protrudes when 

 it is ghding over the mud of the pond. 

 In very vivid words he wrote (1899) : 



' From the functional point of view, the cone of growth 

 may be regarded as a sort of club or battering ram, 

 endowed with exquisite chemical sensitiveness, with rapid 

 amoeboid movements, and with a certain impulsive force, 

 thanks to which it is able to press forward and overcome 

 obstacles met in its way, forcing cellular interstices until 

 it arrives at its destination '. 



This was in great part a prophetic interpretation, and 

 many have vigorously opposed the conclusion that the 

 development of nerve paths is really due to the protoplas- 

 mic movement on the part of the nerve-cells. But brilliant 

 confirmation of Ramon y Cajal's view has been recently 

 afiorded by Professor Ross Granville Harrison, to whose 

 work we wish briefly to refer. 



Harrison's experiments show that two elementary 

 phenomena are involved in nerve development : {a) the 

 formation of the primitive nerve fibre by an outflowing 

 movement of the protoplasm of a nerve-cell, and (b) the 

 formation of neurofibrils within thid filament — a process 

 of tissue differentiation. 



' It is through the former that the specific nerve paths 

 of the body are first laid down '. ' The energy of outgrowth 



