876 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



to the nature of the pineal gland. It appears to possess 

 in all forms an epithelial structure, but, except at the 

 base of the stalk (infra-pineal process) in Mammalia, in 

 the wall of which there are nerve-fibres, no nervous 

 structures are present in it in the adult state. 



The cerebral hemispheres. It will be convenient 

 to treat separately the development of the cerebral 

 hemispheres proper, and that of the olfactory lobes. 



In the cerebral rudiment two parts may be dis- 

 tinguished, viz. the floor and the roof. The former gives 

 rise to the ganglia at the base of the hemispheres, the 

 corpora striata, the latter to the hemispheres proper. 



The first change which takes place consists in the 

 roof growing out into two lobes, between which a shallow 

 median constriction makes its appearance (Fig. 121). 



Fia. 121. 



-ce 



S.v 



-op.f/i 



Diagrammatic Longitudinal Horizontal Section through 

 THE Fore-brain. 



Z.v. third ventricle ; Iv. lateral ventricle ; It. lamina terminaUs ; 

 ce. cerebral hemisphere ; o'p.ih. optic thalamus. 



