527 



The determination of the homologies of the various parts of the 

 forebrain in the lowlier Vertebrata is a problem of very great difficulty, 

 which cannot be solved satisfactorily until a considerable amount of 

 careful detailed research has supplied us with a larger body of de- 

 finitely established facts than we possess at present. 



During the last few years many attempts have been made to 

 systematise our knowledge of this part of the brain ; but I think most 

 of the generalisations are not only premature and foredoomed to 

 failure as an explanation of the facts, but are calculated also to hinder 

 the progress of cerebral morphology by introducing serious elements 

 of confusion. 



Yet, in attempting to interpret the homologies of the parts of the 

 forebrain, we have a certain number of clearly-defined guiding marks, 

 the cautious employment of which can afford us considerable assistance 

 in arriving at a reliable conclusion. 



During the past fourteen years I have repeatedly called attention 

 to the means of identifying the hippocampal formation and the parts 

 of the hemisphere bordering on the lamina terminalis (especially the 

 paraterminal body) and have insisted on the great value of the clue 

 to the interpretation of the rest of the hemisjihere, which the recognition 

 of these parts affords. Yet in several recent works these criteria of 

 cerebral homologies have been set at nought and in several cases the 

 authors of the works in question have returned to the errors of fifteen 

 years ago. 



I have discussed the whole subject at considerable length so re- 

 cently as six years ago, — and during the time that has elapsed since 

 then nothing has been brought to light to throw any real doubt upon 

 my facts or arguments — that it is surely unnecessary to repeat or 

 even to recapitulate what I stated then ^). It would serve no useful 

 purpose to criticise seriatim all the recent works just referred to, in 

 which no reasons are given for adopting views regarding the morpho- 

 logy of the cerebral hemisphere in non-mammalian vertebrates other 

 than that based upon the recognition of the hippocampus and para- 

 terminal body (by the establishment of the identity of the relations 

 and the structure of these bodies with those of foetal acallosal mam- 

 malian brains). 



1) On the Morphology of the Cerebral Commissures in the Verte- 

 brata, with ISpecial Reference to an Aberrant Commissure found in 

 the Forebrain of Certain Reptiles. Transact, of the Linnean Society, 

 London, Ser. 2, Zoology, Vol. 8, 1902, Pt. 12. 



