452 SPINAL NERVES. 



Returning to the original attachment of the nerve-rudiments 

 to the medullary wall, it has been already stated that this 

 attachment is not permanent. It becomes, in fact, at about the 

 time of the appearance of the above commissure, either extremely 

 delicate or absolutely interrupted. 



The nerve-rudiment now becomes divided into three parts 

 (figs. 267 and 268), (i) a proximal rounded portion, to which is 

 attached the longitudinal commissure (pr) ; (2) an enlarged 

 portion, forming the rudiment of a ganglion (g and sp g)\ (3) a 

 distal portion, forming the commencement of the nerve (it). 

 The proximal portion may very soon be observed to be united 

 with the side of the spinal cord at a 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 succeeding 

 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 1 . 



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

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

 that His and Kolliker, following Remak 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 histogenesis of nerves, but 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 investigations 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. 



