Functional factors in the morphology of the forebrain of fishes 185 



hippocampus at all, but with the lobus pyriformis and underlying 

 amygdala. 



All parts of the teleostean área olfactoria dorsalis are connected with 

 the hypothalamus. Most of these fibers take the lateral course of the 

 primitive tractus pallii, but some are dispersed throughout the entire 

 thickness of the wail. The more medial fibers pass down through the me- 

 dial forebrain bundle (fig. 24), but, asjohnston has pointed out, these are 

 not to be identified with the mammalian columna fornicis, ñor is the dorso- 

 medial tissue with which they connect a specific primordium hippocampi. 

 This dorso-medial región (figs. 23, 24, a. ol. d. ni.) has a distinctive histo- 

 logical structure which is probably determined largely by its functional 

 relations with the medial olfactory área. The same is true of the amphi- 

 bian primordium hippocampi, but the two structures are not morpholo- 

 gically exactly equivalent. The medial part of the área olfactoria dorsa- 

 lis of Acanthias also has a medial hypothalamic connection termed for- 

 nix by Johnston. These fibers pass down in front of the foramen — not 

 behind it, as in teleosts — and probably form a true columna fornicis. 



Again, the peculiar structure of the dorso-lateral tissue of Amia and 

 teleosts (figs. 23, 24, a. ol. d. di.) is probably determined by its functional 

 relations with the underlying área olfacto-somatica. If, as Sheldon main- 

 tained, the área olfacto-somatica receives somatic sensory fibers from the 

 thalamus, then the área superficial to it probably also participates in the 

 olfacto-somatic reactions, for all agree that these regions are histologi- 

 cally intimately related. This is a movement in the direction of the dif- 

 ferentiation of the general cortex and would seem to aflbrd strong sup- 

 port to Holmgren's view that this sector is in reality primordial general 

 cortex. This may be true, but I incline to doubt the propriety of attemp- 

 ting to localize the various subdivisions of reptilian or mammalian cere- 

 bral cortex at all in the brains of these fishes. Wliat we have in teleosts 

 particularly are local functionally determined ditíerentiations which are 

 probably of late phylogenetic origin (cenogenetic, not palingenetic). 



Our conclusión is that the embryological development does not sup- 

 port Mrs. Gage's theory of the mechanics of eversión in the telencepha- 

 lon of teleosts and that the functional connections of the parts of the 

 everted área olfactoria dorsalis of the adult speak decisively against the 

 views of cortical homologies oí the various parts of this área wliich 

 have been based upon her theory. The área as a whole has given rise to 



