210 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



(2) The Choroid or Transverse Fissure (Fig. 171), caused by 

 the inflection of the mesial wall of the vesicle on the velum 

 interpositum and choroid plexuses. 



sept, lucid. 4 

 rostrum 



ant. comm. 

 lamina cinerea 



corp. eallos. 



trans, fis. 



callosal sulcus 



gyrus subcallos. 

 optic, chiasmd 



supra-callos. gyrus 

 -fornix 



\transu. or choroid, fis. 



fimMa \ippocamp.fis. 

 /uncus 2 fas. dentata 



attach, of crura cerebri 



Fig. 172. — Diagram to 6how the structures formed in tlie Lamina Terminalis and 

 Primitive Callosal Gyrus. (After Elliot Smith.) 



(3) The primitive callosal gyrus 1 (see Figs. 171 and 172) is that 

 part of the mesial wall of the cerebral vesicle which lies between 

 the arcuate and transverse (choroid) fissures. In the lower edge 

 of this marginal gyrus, the edge which bounds the transverse 

 fissure, and therefore overlies the velum interpositum, is developed 

 the Fornix with the fimbria, its posterior continuation, a longi- 

 tudinal commissure which connects the optic thalamus with the 

 hippocampal (uncinate) gyrus. The corpus callosum is developed 

 in the lamina terminalis above the foramen of Monro about the 

 end of the 3rd month. It afterwards extends backwards, en- 

 croaching on and displacing the primitive callosal gyrus. 



The grey matter of the primitive callosal gyrus becomes 

 reduced to — (1) The vestigial supra-callosal gyrus, lying on the 

 upper surface of the corpus callosum (Fig. 172); (2) the gyrus 

 dentatus ; (3) the gyrus, which unites 1 and 2 round the 

 splenium of the corpus callosum ; (4) the cortex of the hippo- 



1 This gyrus forms part of the Bhinencephalon, and the name is proposed 

 merely as a provisional one, until comparative anatomists agree as to its proper 

 designation. At present, Elliot Smith proposes the term "hippocampal forma- 

 tions " for the parts of the brain derived from it in the adult. 



