100 THE BRAIN OF THE TIGER SALAMANDER 



animals seems consonant with this interpretation; and in Aronson 

 and Noble's study of the sexual behavior of frogs this facilitating 

 action of the olfactory field is clearly demonstrated. 



In primitive vertebrates the dominance of the entire anterior end 

 of the brain by the olfactory apparatus implies more physiological 

 homogeneity than in higher brains, where this tissue is invaded by 

 larger numbers of nonolfactory fibers with more diverse specificities. 

 The case is somewhat like the invasion of a hitherto isolated conti- 

 nent with homogeneous and primitive population by immigrants of 

 numerous other races with very diverse cultural standards. When 

 European peoples colonized North America, in some regions the 

 newcomers intermarried with the natives and the two races amalga- 

 mated; in other places the indigenous population was driven farther 

 and farther back or exterminated altogether. Something analogous 

 to these processes has taken place during the invasion of the olfactory 

 area by nonolfactory functional systems. In some regions there is 

 blending of the old and the new, as in the amygdala, septum, and 

 olfactory tubercle; in other places the indigenous olfactory system 

 has more nearly retained its unmixed character, as along the margin 

 of the olfactory bulb; and in other extensive regions of the hemi- 

 sphere the indigenous elements have been almost entirely displaced 

 by nonolfactory systems, as in part of the corpus striatum and the 

 neopallial cortex. In the Amphibia this invasion of the olfactory field 

 by nonolfactory systems is extensive, but the invading forces are not 

 sufficiently diversified and localized to invoke the differentiation of 

 cortical tissue in the pallial part of the hemisphere. This is probably 

 correlated with the fact that amphibian behavior, by and large, is 

 mass movement, with relatively little refined analysis into partial 

 patterns. 



The primitive differentiated cortex of reptiles has three well- 

 defined sectors. These are spread, respectively, on the dorsomedial, 

 dorsolateral, and dorsal convexities of the hemisphere. The first is 

 archipallium, the precursor of the mammalian hippocampal forma- 

 tion. The second is paleopallium, or piriform cortex, represented in 

 man by a relatively small area at the lower border of the temporal 

 lobe and including the uncus and some adjoining areas. The third 

 sector, the dorsal cortex, is of uncertain relationships. It occupies the 

 position of the neopallium, which comprises the larger part of the 



