NERVOUS SYSTEM OF THE VERTEBRATA. 423 



In the case of the fore-brain the earliest histological changes, 

 except possibly in Mammals, take place on the same general 

 plan as those of the remainder of the central nervous system 1 ; 

 but though the general plan is the same, yet the early histologi- 

 cal distinction between the fore-brain, and the mid- and hind- 

 brain is more marked than the distinction between the latter 

 and the spinal cord. 



On the floor and sides of the thalamencephalon, and appa- 

 rently the whole of the hemispheres of the lower types, there is 

 formed, somewhat later than in the remainder of the brain, a 

 very delicate layer of white matter. The inner part of the wall, 

 which still remains comparatively thin, is not at first clearly 

 divided into an epithelial and nervous layer. This distinction 

 soon however becomes more or less apparent, though it is not so 

 marked as in most other parts of the brain ; and it appears that 

 in the subsequent growth the greater part of the original 

 epithelial layer becomes converted into nervous tissue. 



In Mammals the same plan of differentiation would seem to 

 be followed, though somewhat less obviously than in the lower 

 types. The walls of the hemispheres become first divided 

 (Kolliker) into a superficial thinner layer of rounded elements, 

 and a deeper and thicker epithelial layer, and between these the 

 fibres of the crura cerebri soon interpose themselves. At a 

 slightly later period a thin superficial layer of white matter, 

 homologous with that of the remainder of the brain, becomes 

 established. 



The inner layer, together with the fibres from the crura 

 cerebri, gives rise to the major part of the white matter of the 

 hemispheres and to the epithelium lining the lateral ventricles. 



The outer layer of rounded cells becomes divided into 

 (i) a superficial part with comparatively few cells, which, 

 together with its coating of white matter, forms the cortical 

 part of the grey matter, and (2) a deeper layer with numerous 

 cells which forms the main mass of the grey matter of the 

 hemispheres. 



The development of the several parts of the brain will now 

 be described. 



1 I have worked out these changes in Elasmobranchii, Amphibia (Salamandra) 

 and Aves. 



