Fuiíctional factors in thc inorpholo^y of the forebrain of lishes 195 



fluencecl thereby. Ascending and descending fibers which cross the di- 

 telencephalic boundary are so arranged as to intégrate the di-telencepha- 

 lic centers ¡n three longitudinal columns or zones which converge toward 

 the bulbus olfactorius. These three zones are purely olfactory in front; 

 posteriorly they are invaded by various non-olfactory systems ascending 

 from the diencephalon. Local differentiation within the telencephalon 

 médium occured in response to the diverse physiological characteristics 

 of these ascending systems, and also in correlation with the develop- 

 ment of systems of correlation neurons intrinsic to the telencephalon 

 itself. 



The hypothalamus is the most important diencephalic correlation cen- 

 ter of fishes. The ventral part of the telencephalon médium is adjacent 

 to it and intimately connected with it by the medial forebrain bundle. 

 These physiologically related regions torm the ventral zone: área olfacto- 

 ria medialis, nucleus preopticus, hypothalamus. 



The thalamus is devoted to exteroceptive or somatic sensori-motor 

 correlation— optic, somesthetic, acoustico-lateral systems. The adjacent 

 part of the telencephalon médium (área somática) is of like physiological 

 character. The intermedíate zone includes the área oltactoria lateralis, 

 área olfacto-somatica, área somática, and thalamus, purely olfactory in 

 front and purely somatic behind. These parts are connected by ascend- 

 ing and descending longitudinal fiber tracts which are the precursors of 

 the thalamic projection fibers of higher forms. Within this zone is an 

 olfactostriatal primordium, from which the amygdalo-striate complex has 

 been differentiated. 



The epithalamus was probably primitively an olfacto-tectal and olfacto- 

 thalamic correlation center. The adjacent part of the telencephalon is 

 the área olfactoria dorsalis or primordium hippocampi and these two 

 regions form the dorsal zone. Early in phylogeny the somatic sensory 

 components of this zone were largely taken over by the intermedíate 

 zone. The área olfactoria dorsalis of fishes, accordingly, is mainly de- 

 voted to olfacto-hypothalamic correlation, but of different physiologic 

 type from that of the ventral zone. ít is connected with other parts of 

 the telencephalon by extensive systems of correlation fibers and is clearly 

 the most highly specialized part of the telencephalon. 



The preceding summary relates to the more generalized ganoid fishes. 

 In more specialized forms iteleosts and elasmobranchsi the same physio- 



