58y 



I refer to the matter now, merely to point out that the same deter- 

 mining cause which in the Eutheria calls the "corpus callosum" into 

 being is probably functional in bringing into existence the "aberrant 

 bundle" in the Diprotodontia. 



When the relations of these commissural bundles in the four diver- 

 gent mammalian groups — Monotremata, Pohjprotodontia, Diproto- 

 dontia, and EutJieria — are carefully studied we are able to appreciate 

 one — and by no means the least — of the reasons why the Eutheria 

 have attained such a pronounced ascendency over the other three 

 groups. 



Their brain is that which has retained that particular modification 

 of the commissural arrangement which not only furnishes the shortest 

 and most direct path of communication [c] between the two hemi- 

 spheres, but also permits of an unimpeded expansion (which is so freely 

 exercised by the corpus callosum). In the other three groups, in which 

 all the neopallial commissural fibres pass through the ventral commis- 

 sure, the undue expansion of the latter would produce considerable 

 disturbance in the surrounding structures, which in turn would exer- 

 cise a restraining influence upon any marked increase in size in the 

 commissure itself. 



The development of any such commissural mass as the corpus cal- 

 losum of the more highly organised Mammalia in the position occupied 

 by its homologous fibres (fig. 3, a and b) in the Monotremes and 

 Marsupials would cause the most profound disruptions of the corpus 

 striatum, optic thalamus, and the basal region of the brain, and the 

 complete disorganisation of its whole. 



For these various reasons the development of the corpus callosum 

 gives the Eutherian brain a great advantage in the struggle for 

 supremacy, which must have exercised a considerable if not predomi- 

 nant influence in making the EtitJieria the highest Mammals. 



II. Mittheilungen aus Museen, Instituten etc. 



74. Versammlung deutscher Naturforscher und Ärzte in Karlsbad 

 21.— 27. Sept. 1902. 



lu. Abtheilung: Zoologie, einschl. Entomologie. 



Einführende: Dr. Nagl (Karlsbad). Prof. Dr. v. Lendenfeld (Prag). 



Prof. Dr. Cori (Triest). 



Schriftführer: Assist. Freund, Demonstrator Mascha (Prag). 



Sitzungslocal: Gymnasium, I. Stock, Tertia. 



Frühstücks- und Mittagslocal : »Bayrischer Hof«. 



