788 
ROSS GRANVILLE HARRISON 
forth the hypothesis of chemotaxis to account for these supposed 
movements. The discovery, by the same observer ('90), of the 
cdnes d^accroissement, found at the end of embryonic nerve fibers 
very early in their development, had given a clue as to what this 
growth mechanism might be, for the resemblance of the minute 
processes borne upon the terminal enlargement of the growing 
nerve to pseudopodia, naturally suggested that this structure 
might owe its peculiarities to amoeboid activity. In his larger 
work on the structure of the nervous system Cajal ('99) elabo- 
rates his theory more fully and leaves no doubt as to his meaning 
regarding the activity of the growth cones. After describing 
their appearance he says (p. 544-5) : ^'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." From this it is seen that Ramon y Cajal 
took a considerable step in advance of His ('86-'90), and placed 
upon a still firmer basis the concept that the nerve fiber is formed 
as the outgrowth of a single cell. 
Although this view has enjoyed wide acceptance, the opposing 
theory of Hensen ('64-'08), which denies that there is a free 
outgrowth of protoplasmic substance to form the nerve fibers, 
has met with increasing support within the past few years, es- 
pecially in the work of 0. Schultze C04-'08), Braus ('04-05), 
Held ('06-'09), Paton ('07) and Schaeppi ('09); and it seems 
that we are really very far from a satisfactory solution of the 
question, which even the invention of new and marvelousl}' 
refined histological methods has failed to bring to a final settle- 
ment. Nor has Held's- compromise theory, which is based upon 
such methods, and which sees in Hensen's protoplasmic bridges 
merely a sort of substratum into which the fibrillar substance ex- 
tends from the neuroblasts or ganglion cells, succeeded in har- 
\ Held's view appears on the surface to be a modification of Hensen's theory and it 
is usually classed as such, but a full examination of his complete work shows that 
in reality it approaches much more closely to His's view. 
