880 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



The outer wall of the hemispheres gradually thick- 

 ens, while the inner wall becomes thinner. In the 

 latter, two curved folds, projecting towards the interior 

 of the lateral ventricle, become formed. These folds 

 extend from the foramen of Monro along nearly the 

 whole of what afterwards becomes the descending cornu 

 of the lateral ventricle. The upj)er fold becomes the 

 hippocampus major (cornu ammonis) (Figs. 123 amm, 

 124 and 125 h, and 126 am). 



The wall of the lower fold becomes very thin, and a 

 vascular plexus, derived from the connective-tissue 

 septum between the hemispheres, and similar to that of 

 the roof of the third ventricle, is formed outside it. It 

 constitutes a fold projecting into the cavity of the 

 lateral ventricle, and together with the vascular con- 

 nective tissue in it gives rise to the choroid plexus of 

 the lateral ventricle (Figs. 124 and 125 pi). 



It is clear from the above description that a marginal 

 fissure leading into the cavity of the lateral ventricle 

 does not exist in the sense often implied in works on 

 human anatomy, since the epithelium covering the 

 choroid plexus, and forming the true wall of the brain, 

 is a continuous membrane. The epithelium of the 

 choroid plexus of the lateral ventricle is quite inde- 

 pendent of that of the choroid plexus of the third 

 ventricle, though at the foramen of Monro the roof of 

 the third ventricle is of course continuous with the 

 inner wall of the lateral ventricle (Fig. 124 s). The 

 vascular elements of the two plexuses form however a 

 continuous structure. 



The most characteristic parts of the Mammalian 

 cerebrum are the commissures connecting the two 



