212 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



striatum comes to be applied to that body. The posterior 

 extremity of the cerebral vesicle, in which the tail of the caudate 

 nucleus is situated (Fig. 173), develops downwards, behind and 

 below the optic thalamus, forming the descending horn of the 



occip. region 

 caud. nucleus I temp, lobe 



cereb. \iesic.—/V-. 



front, lobe 

 for. of Monro 

 eorp. striat. 



olf. lobe 



lam. te'rmin. 



hypothal. region 



pituitary 3r d ventricle 



Fig. 173.— Showing the Development of the Corpus Striatum in the floor and outer 

 wall of the Cerebral Vesicle. 



lateral ventricle. Hence the occurrence of the tail of the caudate 

 nucleus and the amygdaloid nucleus on the roof of the descending 

 horn, both being intrinsic parts of the corpus striatum. The 

 corpus striatum is imperfectly separated, during the 4th and 

 5th months of foetal life, into two parts, the caudate and 

 lenticular nuclei, by the downgrowth, from the cerebral cortex 

 of the fibres which form the internal capsule, crusta, and pyra- 

 midal tracts. 



Formation of the Island of Reil and Fissure of Sylvius. — 

 The formation of the corpus striatum, the Island of Eeil, and 

 Fissure of Sylvius are part and parcel of the same process. 



When the lateral wall of the cerebral vesicle is examined at 

 the 5th month (Fig. 175) an area of cortex is seen to be rapidly 

 becoming submerged by the overgrowth of the surrounding 

 cortex. The submerged area is the Island of Eeil ; it covers 

 that part of the wall of the cerebral vesicle which is thickened 

 by the corpus striatum (Fig. 168). The submerged area becomes 

 triangular in shape, the apex being directed backwards ; it is 



