378 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



FIG. 122. 

 / 2. 



BRAIN OF A THREE MONTHS' HUMAN EMBRYO : NATURAL SIZE. 

 (From Kolliker.) 



1. From above with the dorsal part of hemispheres and mid- 

 brain removed ; 2. From below. /. anterior part of cut wall 

 of the hemisphere ; /'. cornu ammonis ; tho. optic thalamus ; 

 cst. corpus striatum ; to. optic tract ; cm. corpora mammil- 

 laria ; p. pons Yarolii. 



the floor becomes thickened on each side, and gives rise 

 to the corpus striatum (Figs. 124 and 125 st}. The 

 corpus striatum projects upwards into each lateral ven- 

 tricle, and gives to this a somewhat semilunar form, the 

 two horns of which constitute the permanent anterior 

 and descending cornua of the lateral ventricles (Fig. 126 

 st). 



With the further growth of the hemisphere the cor- 

 pus striatum loses its primitive relations to the de- 

 scending cornu. The reduction in size of the foramen 

 of Monro 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 thalami. In the later stages of 

 development the area of contact between these two 

 pairs of ganglia increases to a large extent (Fig. 125), 



