ii NERVOUS SYSTEM 121 



While the present writer is inclined to believe that the junction 

 is already in existence while end-organ is still in close apposition to 

 the central nervous system there is no difficulty in principle in the 

 way of admitting that the bridge may in certain cases be formed 

 somewhat later, as Dohrn describes, provided always that the gap to 

 be bridged over is small and the bridge itself protoplasmic and not 

 fibrillar. It is probably along such lines that we may look for a 

 reconciliation between the supporters of His (the outgrowth view) and 

 those who believe in the protoplasmic bridge view but it will involve 

 dropping what are essential features in the outgrowth view as enunci- 

 ated by His himself — (1) that the outgrowth arises at a time when 

 the end-organ has already retreated to a considerable distance from 

 the nerve-centre and (2) that the outgrowth is already fibrillated dur- 

 ing the outgrowing process and before it is united to its end-organs. 



Spinal Ganglia and Dorsal Eoots. — As has already been in- 

 dicated, the. central nervous system of the Vertebrate consists in its 

 most primitive condition of a specialized area of the ectoderm of the 

 dorsal surface. It is further very characteristic of the Vertebrate 

 that those ganglion-cells which belong especially to the sensory fibres 

 have become concentrated into segmentally arranged clumps towards 

 the margin of the nervous plate and have eventually come to lie out- 

 side the limits of the actual plate, or tube into which the plate be- 

 comes converted. These little detached pieces of the central nervous 

 system are the ganglia of the dorsal roots or the spinal ganglia. 1 



During actual ontogeny the ganglion rudiments in some cases 

 {e.g. Birds, Eig. 67, A) become distinctly apparent while the spinal 

 cord is still in the form of an open medullary plate. They appear 

 in the form of a continuous proliferation from the inner surface of 

 the ectoderm in the angle between the medullary plate and the 

 external ectoderm. In such cases the two rudiments become carried 

 iu towards one another, as the edges of the medullary plate curve 

 inwards to form the neural tube, and undergo fusion across the mesial 

 plane. There is thus formed a median unpaired plate or tract of cells 

 lying just over the roof of the neural tube and between it and the 

 external ectoderm. This is known as the neural crest (Marshall). 



More usually the ganglionic rudiment makes its first appearance 

 after the closure of the neural tube and in such cases the paired stage 

 of the rudiment is slurred over, the neural crest being formed by 

 proliferation of the roof of the neural tube. This is well seen in the 

 case of Elasmobranchs (Fig. 67, B, C). 



However it originated, the plate-like neural crest splits into two 



1 There is reason to believe that this is an instance of a widespread tendency in 

 evolution for groups'of ganglion-cells to undergo gradual shifting towards the direction 

 from which their most frequent afferent impulses come. In other words there is a 

 tendency to shorten the afferent path by shifting the cell-body. This principle of 

 neurobiotaxis has been developed by Ariens-Kappers in his various papers (e.</. 1913). 

 It is particularly conspicuous in the changes which have come about in the position of 

 the ganglionic centres of the various cranial nerves within the brain in the different 

 groups of Vertebrates. 



