372 SPIXAL NERVED. 



very considerable distance from its original point of attachment. 

 Moreover the proximal portion of the nerve is attached, not by its 

 extremity, but by its side, to the spinal cord (fig. 268 x). The dorsal 

 extremities of the posterior roots are therefore free. 



This attachment of the posterior nerve-root to the spinal cord is, on 

 account of its small size, very difficult to observe. In favourable specimens 

 there may however be seen a distinct cellular prominence from the spinal 

 cord, which becomes continuous with a small prominence on the lateral 

 border of the nerve-root near its proximal extremity. The proximal ex- 

 tremity of the nerve is composed of cells, which, by their small size and 

 circular form, are easily distinguished from those which form the suc- 

 ceeding or ganglionic portion of the nerve. This part has a swollen 

 configuration, and is composed of large elongated cells with oval nuclei. 

 The remainder of the rudiment forms the commencement of the true 

 nerve. This also is, at first, composed of elongated cells'. 



It is extremely difficult to decide whether the permanent attachment of 

 the posterior nerve-roots to the spinal cord is entirely a new formation, or 

 merely due to the shifting of the original point of attachment. I am in- 

 clined to adopt the former view, which is also held by Marshall and His, 

 but may refer to fig. 269, shewing the roots after they have become 

 attached to the side, as distinct evidence in favour of the view that 

 the attachment simply becomes shifted, a process which might perhaps 

 be explained by a growth of the dorsal part of the spinal cord. The change 

 of position in the case of some of the cranial nerves is, however, so great 

 that I do not think that it is possible to account for it without admitting 

 the formation of a new attachment. 



The anterior roots of the spinal nerves appear somewhat later 

 than the posterior roots, but while the latter are still quite small. 

 Each of them (fig. 269 or) arises as a small but distinct conical out- 

 growth from a ventral corner of the spinal cord, before the latter has 

 acquired its covering of white matter. From the very first the 

 rudiments of the anterior roots have a somewhat fibrous appearance 

 and an indistinct form of peripheral termination, while the proto- 

 plasm of which they are composed becomes attenuated towards its 



1 The cellular structure of embryonic nerves is a point on which I should have anti- 

 cipated that a difference of opinion was impossible, had it not been for the fact that His 

 and Kolliker, following Eeuiak and other older embryologists, absolutely deny the fact. 

 I feel quite sure that no one studying the development of the nerves in Elasmobranchii 

 with well-preserved specimens could for a moment be doubtful on this point, and I can 

 only explain His' denial on the supposition that his specimens were utterly unsuited to 

 the investigation of the nerves. I do not propose in this work entering into the histo- 

 geiiesis of nerves, hut may say that for the earlier stages of their growth, at any rate, 

 my observations have led me in many respects to the same results as Gotte (Entwick. 

 d. Unke, pp. -482483), except that I hold that adequate proof is supplied by my in- 

 vestigations to demonstrate that the nerves are for their whole length originally formed 

 as outgrowths of the central nervous system. As the nerve-fibres become differentiated 

 from the primitive spindle-shaped cells, the nuclei become relatively more sparse, and 

 this fact has probably misled Kolliker. Lowe, while admitting the existence of nuclei 

 in the nerves, states that they belong to mesoblastic cells which have wandered into the 

 nerves. This is a purely gratuitous assumption, not supported by observation of the 

 development. 



