174 C. Jiídson Herrick 



cending fibers into both bundles but ascending fibers seem to enter only 

 the lateral and dorso-lateral parts. These come from the lateral bundle 

 which therefore contains the tractus pallü of Johnston's descriptions. 



The functional connections of the área olfacto-somatica and área so- 

 mática (so far as known) have been summarized in the description of 

 these regions. The telencephalic somatic sensori-motor correlation sys- 

 tems (thalamic tracts) do not seem to be so well segregated away from 

 the hypothalamic tracts in teleosts as in selachians, and the entire process 

 of forebrain differentiation has apparently taken different directions in 

 these two phyla. 



Summary. — Amia and the Téleoste! differ from the simpler pattern 

 of forebrain morphology of Acipenser chiefly in the enlargement of the 

 área olfactoria dorsalis, its lateral eversión, and more complex internal 

 structure. This differentiation is not in the direction of true cerebral 

 cortex like that of higher vertebrates, but it is on the lower plañe of the 

 simpler reflex apparatus. The specialized regions which are differentia- 

 ted within the área olfactoria dorsalis of teleosts probably are not homo- 

 logous with particular cortical áreas of reptiles, though the área as a whole 

 is clearly primordial archipallium. The relations between diencephalon 

 and telencephalon are expressed schematically in figure 25. 



Review of the fiber tracts. 



The theme of this paper is the analysis of the fore-and-aft conduction 

 systems of the brains of fishes and a consideration of the influence of 

 these systems upon the history of the morphogenesis of the telencepha- 

 lon. In all of these animáis the olfactory system is the dominant phy- 

 siological factor in the telencephalon. Primitively fibers of the tractus 

 olfactorius arising in the olfactory bulb probably reached all parts of the 

 telencephalon and extensive regions of the diencephalon. In some te- 

 leosts such fibers are said to pass directly without synapse to the epitha- 

 lamus and hypothalamus (Holmgren), and I believe that a similar con- 

 nection with the epithalamus prevails in cyclostomes and larval urodele 

 Amphibia. In fishes generally the olfactory fibers of the second order 

 which comprise the tractus olfactorius end for the most part in the telen- 

 cephalon and the descending olfactory impulses are carried to the dien- 



