44 2 THE CEREBRAL HEMISPHERES. 



The outer wall of the hemispheres gradually thickens, 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 Munro along 

 nearly the whole of what afterwards becomes the descending 

 cornu of the lateral ventricle. 



The upper fold becomes the hippocampus major (cornu 

 ammonis) (figs. 259 amm, 260 and 261 h, and 262 am). When 



FIG. 261. TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S EMBRYO 

 OF 2'7 CM. IN LENGTH. (From Kolliker.) 



The section is taken a short distance behind the section represented in fi. 260, and 

 passes through the posterior part of the hemispheres and the third ventricle. 



st. corpus striatum; th. optic thalamus; to. optic tract; t. third ventricle; d. roof 

 of third ventricle; c. fibres of cerebral peduncles; c'. divergence of these fibres into 

 the walls of the hemispheres ; e. lateral ventricle with choroid plexus //; h. cornu 

 ammonis;/". primitive falx; am. alisphenoid; a. orbito-sphenoid ; sa. presphenoid ; p. 

 pharynx ; mk. Meckel's cartilage. 



the rudiment of the descending cornu has become transformed 

 into a simple process of the lateral ventricle the hippocampus 

 major forms a prominence upon its floor. 



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, 



