104 THE BRAIN OF THE TIGER SALAMANDER 



neopallium of mammals, not the more differentiated general cortex 

 of the dorsal convexity of the hemisphere. 



The dorsolateral sector of the anterior olfactory nucleus and the 

 rostral end of the primordial piriform area of Necturus may, accord- 

 ingly, be regarded as critical points in further search for the earliest 

 primordium of the neopallium. This region is related with the pri- 

 mordial general cortex of reptiles, which may be regarded as the 

 precursor of the subiculum and other transitional fields rather than 

 of neopallium, sen.su stricto. The preceding account of the probable 

 history of cortical evolution is drawn largely from Crosby's graphic 

 and discerning analysis published in 1917. 



The structure of the pallial field of Amblystoma and its connec- 

 tions were described in 1927 and subsequently in greater detail in 

 several papers devoted to Necturus ('336, '336?, '34, '34a). In these 

 papers and some earlier publications I commented on the fact that 

 the first well-differentiated cortex appears in reptiles in three clearly 

 defined areas; and the opinion was expressed that a prerequisite for 

 this differentiation is the penetration into the pallial field of thalamic 

 projection fibers in separately localized tracts with different physio- 

 logical properties. This minimal localization of function in the projec- 

 tion systems goes hand in hand with local differentiation in the pal- 

 lium and amplification of the cortical associational connections of 

 these areas. This process of local cortical differentiation continues to 

 advance in complexity of pattern in proportion as the systems of 

 thalamic projection fibers are amplified and diversified. 



This principle of cortical morphogenesis receives its first and clear- 

 est exemplification in the obvious difference in the subpallial connec- 

 tions of the medial and lateral parts of the pallial field, hypothalamic 

 connections predominating medially and thalamic connections later- 

 ally. In Ichthyopsida the hypothalamic influence is much stronger 

 than the thalamic, a relation which is strikingly reversed in higher 

 vertebrates. These diencephalic influences are not sufficient to cause 

 cortical differentiation in the amphibian pallium, though there are 

 some local differences in the three recognizable pallial areas and 

 Soderberg ('22) found clearer evidence of cortical incipience in some 

 early larval stages. Holmgren ('22) found evidence of cortical dif- 

 ferentiation in developmental stages of selachians and some other 

 fishes, and in adult lungfishes a primitive cortex is clearly delami- 

 nated externally of the central gray (Rudebeck, '45). 



