NERVOUS SYSTEM OF THE VERTEBRATA. 363 



loses its primitive relations to the descending cornu. The reduction 

 in size of the foramen of Munro above mentioned is, to a large extent, 

 caused by the growth of the corpora striata. 



The corpora striata are united at their posterior border with the 

 optic thalarni. In the later stages of development the area of 

 contact between these two pairs of ganglia increases to an immense 

 extent (fig. 2G1), and the boundary between them becomes somewhat 

 obscure, so that the sharp distinction which exists in the embryo 

 between the thalamencephalon and cerebral hemispheres becomes 

 lost. This change is usually (Mihalkovics, Kolliker) attributed to 

 a fusion between the corpora striata and optic thalami, but it has 

 recently been attributed by Schwalbe (No. 349), with more proba- 

 bility, to a growth of the original surface of contact, and an accom- 

 panying change in the relations of the parts. 



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. 



Tbe upper fold becomes the hippocampus major (cornu ammonis) 

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



Fig. 260. Tkansvekse section through the beain of a sheep's embkyo 



OF 2-7 CM. IN length. (From Kolliker.) 



The section passes through the level of the foramen of Munro. 



St. corpus striatum; m. foramen of Munro ; ^. third ventricle; jyZ. choroid plexus 



of lateral venlricle; /. falx cerebri; th. anterior part of optic thalamus; ch. optic 



chiasma; 0. optic nerve; c. fibres of the cerebral peduncles; h. cornu ammonis; 



p. pharynx; .ta. pre-sphenoid bone; a. orbito-sphenoid bone; s. points to part of the 



roof of the brain at the junction between tbe roof of the third ventricle and the 



lamina terminalis ; /. lateral ventricle. 



