SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 381 



form the ectoderm. It lies always topographically dorsal (or 

 rostral) to the anterior commissure and in all forms which possess 

 a true fornix and pallial commissures it lies beneath the pallial 

 commissures and between the columns of the fornix. 



The lamina terminalis is the anterior portion of the roof-plate 

 and is formed by the fusion of the lips of the anterior neuropore. 

 It extends from the anterior border of the chiasma ridge to the 

 neuroporic recess and is traversed by the anterior commissure. 

 The lamina supraneuroporica is a thickened portion of the roof- 

 plate extending dorso-caudad from the neuroporic recess and 

 containing pallial commissures in cyclostomes, selachians, some 

 ganoids and teleosts, dipnoi, reptiles and mammals. The tela 

 chorioidea telencephali is the anterior part of the membranous 

 roof of the third ventricle, attached rostrad to the lamina supra- 

 neuroporica, containing within its extent the • paraphysis, and 

 separated from the tela chorioidea diencephali by the velum 

 transversum. The evidence on which these definitions rest has 

 been examined by the writer personally in all classes of verte- 

 brates except the Dipnoi. For information regarding the posi- 

 tion of the pallial commissures in Dipnoi I am indebted to Prof. 

 G. Elliot Smith who states in a letter that the pallial commis- 

 sure in Lepidosiren is disposed as in reptiles. 



GROSS RELATIONS IX THE MEDIAL WALL OF THE HEMISPHERE 



In evaginated brains such as those of the selachians, amphib- 

 ians, reptiles and mammals the hemisphere projects rostrad be- 

 yond the lamina terminalis and possesses a medial wall. Between 

 the apposed medial walls of the two hemispheres is the great 

 sagittal fissure which is closed caudally by the lamina terminalis 

 and lamina supraneuroporica. The lower part of the medial 

 wall (olfactory lobe) undergoes a secondary fusion in most sela- 

 chains. In reptiles and mammals there is only a thickening of 

 the lamina terminalis and of the lamina supraneuroporica related 

 to the anterior and pallial commissures. In selachians these 

 thickenings and the secondary fusions become so extensive that 

 the external neuroporic recess is reduced to a slender canal. 

 Along the line of this canal the medial wall presents a cell-free 



