NERVOUS FIBRES. 149 
white substance is less consistent in the foetus, it separates the 
more readily, and the artificial generation of such globules is 
very easy of observation in foetal nerves. 
The growth of nerves neither proceeds from the circum- 
ference towards the central organs, nor vice versd, but their 
primary cells are included amongst those from which every 
organ is formed, and which, so far at least as their appearance 
is concerned, present no marks by which they can be distin- 
guished from other cells. They are first characterized as 
nerves, when they become arranged in rows and coalesce to 
form a secondary cell. After that coalescence each nervous 
fibre forms a separate cell, which pursues an uninterrupted 
course from the organ, in which its peripheral extremity is 
situated, to the central organ of the nervous system. The 
white substance of nerves does not appear to be formed at 
so early a period in their peripheral extremities, as it is in 
their trunks. The Medizinischen Zeitung for August 1837, 
contains a description which I gave of some nerves from the 
tail of frog’s larvee, which presented an appearance quite dif- 
ferent from ordinary nerves, inasmuch as they had a pale con- 
tour-and no perceptible cavity. They were nerves in an early 
stage, previous to the development of the white substance. 
They represent the only form of nervous matter which we find 
in the tail of very young larve. Some isolated nerves, having 
the ordinary appearance of the dark contours, gradually make 
their appearance, and afterwards increase in quantity; they 
were first observed in the neighbourhood of the muscular fas- 
ciculus which traverses the middle of the tail. The development 
of the white substance appears therefore to advance from the 
trunks towards the circumference. These white fibres become 
more minute and paler towards the periphery. Sometimes such 
a fibre seems to terminate suddenly with even an incomplete 
acumination. But, on a more accurate observation, some ex- 
tremely delicate, very thin filaments are generally seen going 
off from it. The pale immature fibres in the tail of the 
frog’s larvee also subdivide. A question now arises are those 
more minute fibres (which at least present an appearance of 
subdivision) already prepared within an ordinary white primi- 
tive nervous fibre, or are they actual subdivisions? Since each 
nervous fibre is a secondary cell, and retains its character as 
