THE ORIGIN AND SIGNIFICANCE OF CEREBRAL CORTEX 93 



highly vascular chorioid plexuses, the ventricles are dilated, and the 

 walls of the brain are thin. This insures a supply of oxygen to the 

 brain which is adequate for their quiescent existence. The more 

 active fishes living in well-aerated water lack these features and can- 

 not survive in stagnant water. Their brains have solid masses of 

 tissue in great variety of forms and are sensitive to asphyxiation. 



The fossil record shows that amphibians were derived from gen- 

 eralized fishes similar to the living lungfishes. These are mudfishes 

 with enlarged chorioid plexuses and thin-walled widely evaginated 

 hemispheres. There is geological evidence that in early Devonian 

 time, when the amphibians emerged from the water, there was gen- 

 eral continental desiccation. The lakes and streams were drying up, 

 and over extensive areas the freshwater fishes were faced with the 

 alternative of adaptation to drought or extinction. The more highly 

 specialized species perished, but some generalized and sluggish types 

 of mudfishes made successful adjustment. Their extensive chorioid 

 plexuses and widely expanded thin-walled hemispheres had survival 

 value, for so they were tided over the critical period of oxygen de- 

 ficiency during phyletic metamorphosis. These characteristics persist 

 today in all urodeles, most of which have retained the ancestral mode 

 of life. 



In later evolutionary stages the expanded hemispheric vesicles had 

 the further advantage that space is available for further differentia- 

 tion, and especially for spreading out the correlation tissue in thin 

 sheets, an arrangement which seems to be requisite for refinement of 

 adjustment to the spatial relations of things and for high develop- 

 ment of labile, individually modifiable behavior as contrasted with 

 stable, heritable behavior of instinctive type. The interested reader 

 will find further details about the origin and significance of the evagi- 

 nated form of cerebral hemispheres in the two papers cited ('21, 

 '22a) and in chapter xvi of my Neurological Foundations ('24c). 



From the beginnings of differentiation of the evaginated hemi- 

 spheres, their medial and lateral walls have shown striking and inter- 

 esting differences. The telencephalic connections are variously ar- 

 ranged in the different groups of fishes; but in the Amphibia the 

 cerebral hemispheres have acquired the definitive form with connec- 

 tions that are not fundamentally changed in the higher groups. 

 There is no evidence that this arrangement is due to differences in 

 the quality of the olfactory impulses transmitted from the olfactory 



