NERVOUS SYSTEM OF THE VERTEBRATA. 367 



which in many parts of the bruin at any rate, is only to be observed 

 in the early developmental stages, has a simple phylogenetic explana- 

 tion. 



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 epi- 

 dermis. The next stage was the separation of a deeper layer of the 

 epidermis as a layer of ganglion culls 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 in- 

 volution, 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 xn. the primitive division of the 

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



(1) a fore-brain, representing the ganglion of the pra^oral lobe, and 



(2) the posterior part of the nervous axis, consisting of the mid- 

 and hind-brains and the spinal cord. This view of the division of the 

 central nervous system fits in fairly satisfactorily with the facts of 

 development. The fore-brain is, histologically, 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, the nerves of the fore-brain have a different cha- 

 racter to those of the mid- and hind-brain. 



This primitive division of the central nervous system is lost in all 

 the true Vertebrata, and in its place there is a secondary division- 

 corresponding with the secondary vertebrate head into a brain and 

 spinal cord. The brain, as it is established in these forms, is again 

 divided into a fore-brain, a mid-brain and a hind-brain. The fore- 

 brain is, as we have already seen, the original ganglion of the prffioral 

 lobe. The mid-brain appears to be the lobe, or ganglion, of the third 

 pair of nerves (first pair of segmental nerves), while the hind-brain is 

 a more complex structure, each section of which (perhaps indicated 

 by the constrictions which often appear at an early stage of develop- 



