THE CEREBRAL HEMISPHERES 165 



are still more numerous and treacherous, and the most 

 critical students are very conservative about suggest- 

 ing homologies. 



When all of the available evidence is reviewed it is 

 clear that in marsupials, rodents, and other lower 

 mammals cortical differentiation is still in its incip- 

 iency. There is great advance over the reptilian con- 

 dition, where only three cortical areas are clearly evi- 

 dent and these are defined chiefly in terms of their 

 subcortical connections, for the internal structure is 

 very similar in all of them. 



In lower mammals the archipallium has attained 

 its definitive form, which is very complex; but the 

 neopallium is very incompletely differentiated as com- 

 pared with mammals of intermediate grade like the 

 dog, and it is very rudimentary indeed as compared 

 with the human. These facts must be kept in mind 

 throughout the study of the part played by the cortex 

 in behavior patterns in all of these animals. 



