HIPPOCAMPAL F010IATK >.\ 



121 



laiil clown between the mantle zone and the ependymal zone by the development of the fibre-paths 

 conm'fting the thalamus with the hemisphere, and later by the addition of the pyramid-fibres. 



The cleft between corpus striatuni and thalamus persists for a time as a groove 

 in which the stria terminal/is or tcenia semicircularis is formed. During the formation 

 of the internal capsule the lenticular nucleus and claustrum take shape as isolated 

 portions of the striate body. 



The velum interpositum (tela choroidea ventriculi tertii) arises from the vascular 

 connective tissue within the longitudinal fissure. This grows in between the 

 ependymal lamellae of the choroidal fissures to form the choroid plexuses of the 

 lateral ventricles. It also of course covers the optic thalami, and lies upon 

 the ependymal roof of the primitive fore-brain, which it inflects into that cavity 

 to form the choroid plexus of the third ventricle. The ependymal covering of each 

 lateral choroidal plexus is derived from the mesial wall of the hemisphere, while the 



falx 



FK;. 1(55. DIAGRAM OF A TRANSVERSE SECTION OF THE BRAIN TO SHOW THE RELATIONS AND FATE 



OF THE MARGIN OF THE MESIAL WALL OF THE HEMISPHERE. (After His.) 



Th, thalamus ; Cs, corpus striatum. The mesial wall is infolded, and the sunk grey cortex ends 

 in a thickened seam, leaving a free edge of white matter. In the temporal horn the parts are labelled 

 hf, hippocampal fissure ; fa, marginal grey seam ; fi, edge of white substance. 



infrachoroidal lamella is prolonged over, and becomes adherent to, the thalamus 

 (lamina affixa) as far as the stria terminalis, which represents the line of union of 

 hemisphere- wall and thalamus-wall. In this w r ay the anterior part of the thalamus 

 comes to lie in the floor of the lateral ventricle. 



The hippocampal formation and the commissures. If a section 

 of the mesial wall of the hemisphere be examined in the brain of an 

 embryo at the beginning of the third month, it will be found that the cortical 

 ganglionic layer which is forming over the pallial surface ceases before it reaches 

 the choroidal fissure, in which, as we have seen, the ependymal layer is alone 

 present. The reticular layer which in the hemisphere develops between 

 the mantle and the ependymal layers, therefore, comes here to the surface 

 (tig. 165). From frontal lobe to temporal horn the margin of the pallium thus 

 consists of a zone in which the cortical grey layer is present, and a zone in which 



