194 C. fudson Herrick 



Conclusión. 



A review of the morphological and functional relations oftelenceph- 

 alon and diencephalon in several types of fislies and a comparison of these 

 with the Amphibia leads to the conclusión tliat the diverse forms offore- 

 brain in these animáis have been largely determined by the space re- 

 quirements of the centers ot correlation between the olfactory apparatus 

 in front and various non-olfactory systems located in lower levéis of the 

 nervous axis. In all ot these forms tlie telencephalon is dominated by 

 the olfactory system. Fhe primary receptive olfactory center in all ca- 

 ses is evaginated to form a hollo^v olfactory bulb which is the primordial 

 cerebral hemisphere. 



Ihe secondary olfactory center, or área olfactoria, is a región where 

 synaptic junctions occur between the descending olfactory tracts, various 

 non-olfactory systems ascending from the diencephalon, and commissural 

 and other correlation neurons intrinsic to the telencephalon. The fibers 

 whicli descend from this área form various olfacto-diencephalic and ol- 

 facto-mesencephalic tracts which discharge directly or indirectly into the 

 lower motor centers. The physiological characteristics of these descend- 

 ing nervous impulses are generally resultants of the interaction of the 

 olfactory and non-olfactory components of the área olfactoria. 



In generalized torms like Acipenser the relatively small área olfactoria 

 is merely a thickening of the lateral \vall of the neural tube between the 

 lamina terminalis and bulbus olfactorius in front and the di-telencephalic 

 ]:)Oundary behind. This is the telencephalon médium, or primitive end- 

 brain, which is whoUy occupied by the área olfactoria except for a small 

 non-olfactory área somática adjacent to the thalamus. 



Within the área olfactoria local differentiations occur under the in- 

 fluence ol physiologically diverse systems of fibers \\hich grow forward 

 into it from centers of different functional patterns of the diencephalon. 

 These diencephalic centers are grouped in three regions, viz., (l) epitha- 

 lamus, (2) hypothalamus, (31 thalamus, of which the first and second are 

 dominated by the olfactory system and the third by various non-olfacto- 

 ry exteroceptive or somatic sensori-motor systems. 



The evidence indicates that the parts of the telencephalon médium 

 adjacent to these three diencephalic regions have been profoundly in- 



