BRAIN OF VERTEBRA! A. 



507 



remains at a low stage of development, and forms a small lamella, 

 which is raised up, however, in a vertical direction ; in the 

 Chelonii (Fig. 284, A IV) and Crocodilini it is broader, and in the 

 latter it is distinguished by the large size of its median portion. 



fig. 284. Brain of a Ckelonian (after Bojanus). B Of a Bird. Vertical median 



sections. I Prosencephalon. Ill Mesencephalon. IV Metencephalon. V Myelen- 



cephalon. ol Olfactory, o Optic nerve, h Hypophysis, a (in A) connection between 



the two hemispheres of the mesencephalon, c Anterior commissure. 



This condition connects theReptilia with the Aves, which are distin- 

 guished by the great proportionate size of their prosencephalon, the 

 hemispheres of which are often greatly broadened out. They are con- 

 nected by a fine anterior commissure (Fig. 284, B c), and enclose a gan- 

 glionic mass, which projects inwards from the side wall, and convert- 

 ing the primitive cavity into a narrow space which is covered over by 

 the thin-walled roof of the hemispheres, itself forms the largest part 

 of the prosencephalon. These 

 masses may be observed in as 

 low forms as the Amphibia, and 

 in the Eeptilia they are very 

 large (Fig. 286, A st). The small 

 thalamencephalon, which is 

 completely covered over by the 

 hemispheres of the prosence- 

 phalon, has its roof divided. The 

 mesencephalon, which is very 

 large in the embryo, is divided 

 into two pieces, which are pushed 

 down to the sides of the brain 

 (Fig. 285, c), and have the general 

 internal cavity continued into 

 them. The large median portion of the cerebellum is transversely 

 laminated, and, owing to its size, covers over the whole of the 

 myelencephalon. 



§ 382. 



In the Mammalia the brain closely resembles that of the lower 

 forms in its earliest stages only (cf. Fig. 280), for, owing to its special 

 line of differentiation, it is very different to that of Birds and 



Fig. 285. Brain of the Domestic F o w I . 

 A From above, B From below, a Bulla 

 olfactorii. b Hemispheres of the prosen- 

 cephalon, c Mesencephalon, d Meten- 

 cephalon. d' Its lateral parts, e Myel- 

 encephalon (after C. G. Cams). 



