r 4.7 ] 



III. T/ie Oricfin of the Corpus Callostim : <!■ Comparative Study of the Jllpyocampal 

 Mec/ion of the Cerebrum of Marsupialia and certain Cheiroptera. Bij G. Elliot 

 Smith, M.D., Ch.M. [Sydney), " James King " Besearch Scholar of the University 

 of Sydney ; St. John's College, Cambridge. (Communicated by Prof. (i. B. Howes, 

 Sec. Linn. Soc.) 



(Plates 15 & 16.) 



Eciul ilst January, 1S97. 



IT is now a generally recognized fact that the dorsal commissure of the Metatherian is 

 not strictly homologous to the similarly-situated commissure of the Eutherian cerebral 

 hemispheres. For while the dorsal commissure of the Marsupial springs mainly, if not 

 wholly, from that peculiar cortical formation which we know as " hippocampus," its 

 topographical representative in the more highly-organized mammal is derived from a 

 much wider cortical area, of which the hippocampus constitutes a part — and in the great 

 majority of cases only a relatively very small j^art. In other Avords, the dorsal commis- 

 sure of the Metatherian is essentially " hippocampal," while that of the Eutherian 

 cerebrum is partly " hippocampal " and partly " non-hippocampal." These " non- 

 hippocampal " fibres of the dorsal commissure of Eutheria are derived from the general 

 or typical cortex which Sir William Turner calls ''pallium.'" 



It will be found that in higher mammals, such as the Primates, the Cetacea, the 

 Carnivora and Ungulata, the " pallial " element forms the main bulk of the dorsal 

 commissure. But if we compare a large series of mammalian ccrebra we find that, 

 speaking generally, there is a rapid decrease in the extent and importance of this 

 " pallial " factor as we descend the Eutherian scale. The " pallial " or " non-hippo- 

 campal " factor shows a rapid actual decrease, while the hippocampal factor takes a 

 continually increasing share in the constitution of the dorsal commissure. Erom 

 such considerations, and from the knowledge that the " pallial " factor was wanting 

 altogether in the Marsupialia, it seemed likely that some Eutherian form might be 

 found in which the newer " non-hippocampal " element might be so slightly develoiDcd 

 as not to interfere with or destroy the resemblance of its dorsal commissure to that of 

 the Marsupial. And if such a transitional form could be found we ought to be able to 

 decide the exact relation of the newly-developed " pallial " fibres to the pre-existing 

 " hippocampal " fibres, and hence form some accurate conception of the nature of this 

 new commissure, which in its highly-developed state in man we know as the " coipus 

 callosum." 



The desired " connecting links " I have had the good fortune to find in certain 

 Cheiroptera — Nyctophilus Timoriensis and Miniopderus Schreibersii — both of which, but 

 more especially the former, realize the conditions above stated. 



In this contribution I propose to compare the commissures and certain ncighbom'ing 



SECOND SERIES. — ZOOLOGY, VOL. VII. 8 



