498 PEOF. G. ELLIOT SMITH ON THE MORPHOLOGY OF THE 



dorsal mass to the hii^jDOcampus, because its cells are diffusely scattered ; but upon 

 morphological grounds the homology cannot be questioned, and the condition of the 

 hippocampus in Emys and certain other Chelonia indicates that a compact column of 

 cells is not a necessary feature in the j)rimitive hippocampus. 



The morphological jilan of the mesial wall of the cerebral hemisphere is so constant in 

 the higher Vertebratn, in Reptiles, Birds, Monotremes, and Marsupials, and even in the 

 placental Mammals (though disguised by revolutionary changes associated with the 

 advent of the corpus callosum), that this plan must be regarded as the common property 

 of the Vertebrata. The relations of tiie paraterminal body to the lamina terminalis and 

 olfactory bulb places the recognition of this body beyond question ; and the mass which 

 rests upon its upper Ijorder and exhibits traces of a cortical structure (according 

 to Nakagawa, Journal of Morphology, vol. i.), unless the Amphibian tyj^e of brain 

 conforms to some plan utterly different from that which all the Amniota possess in 

 common, must be the hippocampus. 



Osborn and all writers who have come after him have assimied as a postulate that 

 the dorsal commissure and the area from which it springs are the undoubted repre- 

 sentatives of the topographically analogous structures in the E-ejitile. There is no other 

 alternative which can for a moment be entertained, and therefore the amorphous dorsal 

 mass in the mesial wall of the Amphibian hemisphere must be regarded as unquestionably 

 the hippocampus. The question of interpretation once settled, the enormous mass of 

 data concerning this type of brain may be readily interpreted. Three schematic repre- 

 sentations of coronal sections in the precommissural region (figs. 34, 35, 36) in foetal 

 Ornithorhynehus (it might equally be any other Mammalian foetus — vide His, E-etzius, 

 Hochstetter), an adult Reptile and Amphibian, will make the general j)lan clear. 



It is quite unnecessai-y, with this key to the interpretation of the Ichthyopsidan brain, 

 to give an account of all the variations of type which this heterogeneous grou^J of 

 Vertebrates includes. The reader who adopts this suggestion will find ample con- 

 firmation of the constancy of the plan, which the study of the Amniota has revealed, 

 throughout the Vertebrata. The type of brain in the Anura is fuUy described by 

 Gaupp, who adopts, with some relatively slight inaccuracies, the interpretation which 

 T have put forward hei'e {vide Gaupp, oji. cit.). 



The writings of Osborn (Morph. Jahrb., Bd. xii., and Journal of Morphology, vol. ii.), 

 and a large number of memoirs by other American writers, of which Kingsbury's 

 account of the brain of Necturus (Journ. Comp. Neurology, 1895) may be mentioned, 

 as well as Burckhardt's work on Triton and Ichthyophis (Zeitsch. f. wiss. Zool. lii. 

 1891), clearly show the manner in which the type becomes simplified and more 

 fish-like in the Urodeles, producing a condition closely resembling that found in the 

 Dipnoi *. 



Through the kindness of Mr. E. I. Bles, of Cambridge, I have been able to study the 

 condition of the brain in a great many genera of the Urodela, and compare them with the 



* Vide FuUiquet, '• Recherches sur lo Cerveau du Protopterus,'' Eeciieil Zool. Suisse, vol, iii. 18SG : and 

 Burckhardt, ' Das Centralnervensystem von Protopterus,' Berlin, 1892). 



