MORPHOLOGY OF THE FOREBRAIN 451 



the telencephalon fully evaginated. The telencephalon medium 

 is elongated and in its massive floor and side walls are differentiated 

 structures corresponding with the four parts into which we have 

 divided the amphibian hemispheres, the membranous roof of 

 the ventriculus impar being the equivalent of the median and lat- 

 eral plexuses of the urodele hemispheres. In the higher fishes 

 the enlargement of the massive parts has led to their lateral 

 eversion, not to an evagination of the whole wall as in Amphibia. 

 The subdivision of the teleostean "hemisphere," therefore, re- 

 sembles more closely the plan of the diencephalon than that of 

 the telencephalon of Amphibia (see p. 477 and fig. 83), but is in 

 iirportant respects unlike both of them. 



This fundamental difference between the teleostean and amphi- 

 bian types of cerebral hemispheres was first pointed out by Mrs. 

 S. P. Gage ('93) and the idea has since been elaborated by Kap- 

 pers and Edinger. An analysis of the teleostean forebrain will 

 shortly be published by Dr. R. E. Sheldon, in connection with 

 which this problem is fully discussed. Accordingly, no further 

 reference will be made to it here save to add that, though very 

 different morphogenic factors have operated in the evolution 

 of the teleostean brain as compared with elasmobranchs and am- 

 phibians, nevertheless the homologous parts are recognizable. 

 Our conclusions follow, with some important modification in 

 details, the interpretation of Johnston ('10, p. 155). 



The selachians have been recently re-examined by Sterzi and 

 from his descriptions it is evident that in these brains the telen- 

 cephalon medium is extensive. In the lower elasmobranchs the 

 evaginated part contains little except the olfactory bulbs, and even 

 in the higher sharks the remaining parts of the telencephalon are 

 differentiated for the most part as local thickenings of the wall, 

 with a relatively small amount of evagination as compared with 

 the Dipnoi and Amphibia. In the diencephalon of Acanthias 

 one of Sterzi's figures ('09, p. 577, fig. 232) shows the sulci ar- 

 ranged in much the same way as in urodeles. See also Burck- 

 hardt's figure of Scymnus ('07, p. 354, fig. 21). These probably 

 separate homologous parts in amphibians and elasmobranchs, but 

 the determination of this point must await a fuller study of their i 



