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 circumference 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 distinguished 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 larve, which presented an appearance quite different from ordinary nerves, inasmuch as they had a pale contour-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 extremely 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 primitive nervous fibre, or are they actual subdivisions? Since each nervous fibre is a secondary cell, and retains its character as
