Functional factois in thc morphology of the forebrain oí fislics 193 



The amphibian primordium hippocampi belongs in the dorsal zone 

 and the strio-aniygdaloid complex in the intermedíate zone, as these terms 

 are used in our schemata of longitudinal difterentiation in fishes. The py- 

 riform lobe is a structure which is not represented as such in the groups 

 of fishes considered in this paper, for it has been differentiated witiiin 

 the Amphibia from a primordium which in Urodela combines certain 

 features of the área olíactoria dorsalis (notably the tractus pallii), the 

 área olfactoria lateralis, and the área somática — a combination of charac- 

 teristics nowhere shown in the fish types here under consideration. 



The lobus pyriformis of reptiles and mammals retains the mixed or 

 ambiguous character which it exhibits in the Amphibia. Ramón y Cajal's 

 very thorough analysis of this región in the mouse shows that its rostral 

 end is nothing other than nucleus olfactorius lateralis. Farther back it has 

 assumed a definite cortical structure with very complex functional con- 

 nections without losing its character as a secondary olfactory nucleus. 

 The mammalian connections w ith the hippocampus, amygdala, and hy- 

 pothalamus are all foreshadowed in Amphibia. 



The mechanism of evagination of the amphibian cerebral hemisphere 

 is still to be determined. In the examination of a considerable number of 

 models of the brains of larval Amblystoma from the early neural tube 

 stage to the metamorphosis I fail lo find any evidence for an inversión of 

 the dorsal borders of the telencephalon médium (primordium hippocam- 

 pi) comparable with that of Acanthias. On the other hand, the process 

 seems to be a true evagination from the start. The picture, however, is 

 complicated by the precocious development of the large bulbus olfacto- 

 rius which in the early stages of the evagination filis almost the entire la- 

 teral wall of the telencephalon médium with a massive thickening which 

 retards evagination everywhere except at the (morphological) dorso-caudal 

 angle, the future posterior pole of the hemisphere. This thinner región 

 oí the initial evagination includes the whole of the future dorsal wall of the 

 hemisphere, that is, the pars pallialis. 



In Amblystoma the precocious thickening of the región of the future 

 bulbus olfactorius retards the evagination of that part of the wall and the 

 thinner pallial parís lake Ihe lead in Ihis process, but the histological dit- 

 ferentiation of the evaginated parís comes dislinctly laler than that oflhe 

 subpallial parts. The history of the development of the functional con- 

 nections of these parts is now under investigalion. 



H V i)t cu a i t' II Cttjtil '3 



