CEEEBRAL CO.MMISSURES IN THE VEKTEBKATA. 489 



explanation improbable, nor does the liistory of its developfnent in Sphenodon lend any 

 support whatever to such an hypothesis. This explanation would jilso involve the 

 necessity of believing that its fusion with the dorso-anterior extremity of the optic 

 tlialamus is secondary and acquired, and not the persistence of a more primitive 

 condition. Certain other aspects of the in'oblcni will be discussed later. 



But in the meantime tliere are certain facts which must be kept in mind in attempting 

 to explain this phenomenon. 



The ])araterminal body is a ganglionic mass interposed between tlie bnlbus olfactorias 

 on the one hand and the hippocampus on the other. The only libre-tracts which 

 establish any connections with it are derived from these two sources. (For tlie sake of 

 simplicity 1 do not specifically mention the connection with tlie cortex of the 

 tuberculum olfactorium, which is merely an indirect path from the olfactory bvilb.) 



The morpliological relations of the hipjiocampus and paraterminal body are thus most 

 intimate ; the latter not only affords a path for tlie hbres (fornix) of the former, but it 

 forms the bond of union of the liippocanipus with the olfactory region of the brain, of 

 Avhich physiologically the hippocampus is merely an api)endage. Not only so, but a 

 study of the relations of the jjrimordtiim hippocampi to the paraterminal body in the 

 Ichthyopsida lends support to the view that the hippocampus may be nun'ely the 

 specialized upper part of the primitive paraterminal body. 



Whether this be so or not, there can be no doubt that the most intimate relationship 

 exists between the two bodies, and any system of subdividing the cerebral hemisphere 

 which separates hipjjocampus and paraterminal body commits a fundamental error which 

 must render it nugatory as a natural subdivision. 



The evidence afforded by comparative anatomy in the Vertebrate series points to the 

 region upon the dorsal margin of the paraterminal body as the original site of the hippo- 

 campus, and it is conceivable that, as in the process of evolution the hippocampus grows 

 backward with the hemisphere it may, in certain cases, cany with it an extension of the 

 associated paraterminal body. 



The attempt to offer an explanation of the significance of the commissiira ahcrruu>i will 

 involve the discussion of some of the most difficult problems in the whole field of cerebral 

 morphology. For, before it is jDossible to apjirecijite the factors which seem to have 

 determined the development of this commissure in the place where it is found, it will 

 be necessary to consider In-iefly the involved questions : (i) of the relationship of the 

 optic thalamus to the cerebral hemisphere ; and (ii) the nature of the choroid plexus of 

 the lateral ventricle. 



I shall discuss the second problem first, and that in relation to the Monotreme brain, 

 because, while the latter presents all the simplicity of the Sauropsidian condition, the 

 neighbouring r(>gions have attained to the full degree of Mammalian difl'ereuliation, 

 wherefore it is possible to recognize them with certainty. 



The epithelial roof of the forebrain extends in Echidna (as in Oi-nithorhynchiis, vide 

 siqyra) from the commissura superior posteriorly to the antero-superior lip of the 

 thickened upper extemity of the lamina terminalis (see fig. 23). The relations of this 

 roof in the region of the recessus superior have already been considered. 



71* 



