Functional factors in the m()rpholo<rv of the forebrain of fishes 



145 



Already in Acipenser the bulbus olfactorius has attained form and 

 internal structure which are preserved ¡n essential features throughout 

 the vertébrate pliykim. But the olfactory correlation centers of the 

 telencephalon undergo a remarkable series of structural and functional 

 changes in different groups of vertebra- 

 tes which are, in fact, the fundamental 

 factors in shaping the course oí ditferen- 

 tiation of the cerebral hemispheres of 

 higher forms. This series of changes is 

 the theme of the present inquiry. 



The peripheral oltactory neurons ot the 

 first order end in the bulbus olfactorius, 

 which is the primary olfactory center, 

 that is, the point of first synapse in the 

 olfactory pathway. The neurons of the 

 second order arise in the formatio bulba- 

 ris and in tlie aggregate their fibers form 

 the tractus olfactorius. The área olfacto- 



VENTRlCut 



Fig. I. — Diagrammalic longitudi- 

 nal section of the forebrain of the 

 sturgeon, Acipenser. The cere- 

 bral hemispheres comprise only 

 ria is the secondary olfactory center, that the olfactory bulbs, the remain- 



der of the endbrain (telencepha- 



I 



on médium or primitive end- 

 brain) being un evaginated but 

 thickened laterally. In figures i, 

 7, 12, 19 and 30 the cut surfaces 

 of the walls of the neural tube 

 are conventionalJv marked as 

 follows: primitive endbrain (te- 

 lencephalon médium), diagonal 

 Unes; evaginated telencephalon 

 (cerebral hemispheres), horizon- 

 tal lines; diencephalon, vertical 

 lines; mesencephalon, unshaded. 



is, the área of distribution of the tractus 

 olfactorius. This área is variously sub- 

 divided and the fibers of the third order 

 which leave these subdivisions can best 

 be named by compound words expres- 

 sing the terminal connections of the 

 tracts, such as tractus olfacto-habenularis, 

 tractus olfacto-mamillaris, etc. In Aci- 

 penser, as in many other fishes, the área 



olfactoria lies wholly in the telencephalon médium; no part of it is re- 

 presented in the hollow evaginated cerebral hemispheres. 



Johnston has given detailed accounts of the internal structure of the 

 brain of Acipenser, from which a lew features are selected for the pur- 

 poses of this review. Some of Johnston's ñames of parts are changed to 

 conform to the nomenclature used throughout this paper and the morpho- 

 logical interpretation is in some details new; but the conclusions reached 

 are (it is believed) in harmony with the general results of his compre- 

 hensive researches. 



Homenaje a Cajal 



