348 HTSTOGENESIS OF THE BRAIN. 



innermost epithelial layer lining the cavities of the ventricles, and an 

 outer layer of grey matter. 



The similarity of the primitive arrangement and histological 

 character of the parts of the brain behind the cerebral hemispheres 

 to that of the spinal cord is very conclusively shewn by the examina- 

 tion of any good series of sections. In both brain and spinal cord 

 the white matter forms a cap on the ventral and lateral parts con- 

 siderably before it extends to the dorsal surface. In the medulla 

 the white matter does not eventually extend to the roof owing to 

 the peculiar degeneration which that part undergoes. 



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 histological distinction 

 between the fore-bra.in, 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 apparently 

 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 re- 

 mainder 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 (1) 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. 



The hind-brain. The hind-brain is at first an elongated, fuunel- 



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

 Aves. 



