Functional factors in the niorphology of the forcbrain of fishes 189 



The evaginated hemisphere of Urodela is obscurely divided into four 

 quadrants or sectors. I hese are much more clearly defined in Anura. 

 The ventro-medial quadrant is the área olfactoria mediahs (septum com- 

 plex); it is dominated by the medial íorebrain bundle. The ventro-lateral 

 quadrant contains the ventral part of the área olfactoria lateralis and the 

 área olfacto-somatica. It is dominated by the lateral forebrain bundle. 

 These ventral parts are clearly subpallial, though fibers from their basal 

 forebrain bundles reach across the zonae limitantes into the overlying 

 dorsal quadrants. Tlie dorso-medial quadrant is the primordium hip- 

 pocampi and is clearly related with the área olfactoria dorsalis of fishes. 

 Ihe dorso-lateral quadrant is the primordial lobus pyriformis and is 

 apparently derived from the dorsal part of the área olfactoria lateralis of 

 fishes. 



The functional connections of the área olfactoria medialis, nucleus 

 preopticus, and hypothalamus are essentially as in fishes, as are also their 

 topographic relations except for the evagination forward of the área olfac- 

 toria medialis to form the septum. This is the ventral zone as defined 

 in fishes. 



The dorso-medial quadrant of the amphibian hemisphere is the most 

 highly specialized portion and is clearly the primordium of the hippo- 

 campal cortex. ()n this point there is general agreement. It is evidently 

 a derivative of the área olfactoria dorsalis of primitive forms. Its physio- 

 logical character, however, is profoundly modified, as compared with the 

 dorsal área of any of the fishes which \ve have been considering. It recei- 

 ves relatively few fibers from the tractus olfactorius and (so far as knovvn) 

 has no connection with the hypothalamus by the lateral olfactory pathway 

 of tractus pallii type. The hypothalamic connection is a inedia! efferent 

 projection tracts, the columna fornicis. There is a connection with the 

 habenula through the stria medullaris. Its most extensive connections 

 are with other parts of the hemisphere by association fibers — with the 

 septum and the lobus pyriformis. The primordium hippocampi clearly 

 belongs in the dorsal zone as defined above in fishes. 



The lateral wall oi the amphibian hemisphere presents a number of 

 difficult problems. In Anura it is clearly separated into dorsal and ven- 

 tral quadrants, but in larvae of these forms and in the lower Urodela the 

 limits betueen these parts fade away, and it is in these more generalized 

 forms that we must seek the answers to our questions. Here the lateral 



