446 GENERAL CONCLUSIONS. 



histological arrangement, which in many parts of the brain at 

 any rate, is only to be observed in the early developmental 

 stages, has a simple phylogenetic explanation. 



As has been already explained in an earlier part of this chapter 

 the central nervous system was originally a differentiated part of 

 the superficial epidermis. 



This differentiation (as may be concluded from the character 

 of the nervous system in the Ccelenterata and Echinodermata) 

 consisted in the conversion of the inner ends of the epithelial 

 cells into nerve-fibres ; that is to say, that the first differentiation 

 resulted in the formation of a layer of white matter on the inner 

 side of the epidermis. The next stage was the separation of a 

 deeper layer of the epidermis as a layer of ganglion cells from 

 the superficial epithelial layer, i.e. the formation of a middle 

 layer of ganglion cells and an outer epithelial layer. Thus, 

 phylogenetically, the same three layers as those which first make 

 their appearance in the ontogeny of the vertebrate nervous system 

 became successively differentiated, and in both cases they are 

 clearly placed in the same positions, because the central canal of 

 the vertebrate nervous system, as formed by an involution, is at 

 the true outer surface, and the external part of the cord is at the 

 true inner surface. 



It is probable that a very sharp distinction between the white 

 and grey matter is a feature acquired in the higher Vertebrata, 

 since in Amphioxus there is no such sharp separation ; though 

 the nerve-fibres are mainly situated externally and the nerve-cells 

 internally. 



As already stated in Chapter XII. the primitive division of 

 the nervous axis was probably not into brain and spinal cord, 

 but into (i) a fore-brain, representing the ganglion of the prae- 

 oral lobe, and (2) the posterior part of the nervous axis, consist- 

 ing of the mid- and hind-brains and the spinal cord. This view 

 of the division of the central nervous system fits in fairly satis- 

 factorily with the facts of development. The fore-brain is, histo- 

 logically, more distinct from the posterior part of the nervous 

 system than the posterior parts are from each other; the front 

 end of the notochord forms the boundary between these two parts 

 of the central nervous system (vide fig. 253), ending as it does at 

 the front termination of the floor of the mid-brain, and finally, 



