236 JOHNSTON. [VOL. I. 



It seems probable that at least some of the cells here 

 described are associational cells serving to coordinate the 

 motor impulses descending from the two halves of the fore- 

 brain, while other cells may belong to the olfactory area and 

 give rise to a part of the tractus olfacto-habenularis. The 

 disposition of the bundles of the tractus olfacto-habenularis, to 

 be noted below, suggests homology of the cells just described 

 with the cortex lateralis of reptiles, to which they correspond 

 in position. In the choroid roof of the fore-brain in Acipenser 

 I have had one or two nerve cells and fibers impregnated in a 

 few preparations. These cells correspond in position with the 

 cortex dorsalis of higher forms. 



(5) Nucleus tJiacniac. The cells surrounding the recessus 

 praeopticus and the ventral portion of the central cavity of the 

 fore-brain as far forward as the anterior commissure constitute 

 a nucleus corresponding to the nucleus occipito-basalis of 

 Herrick ('91) and the nucleus thaeniae of Edinger ('96a). Their 

 dendrites spread widely toward the lateral surface of the fore- 

 brain. At least some of their axis cylinders run backward 

 through the optic chiasma; I have been unable thus far to trace 

 them to their endings. There is a decussation of some of the 

 fibers beneath the recessus praeopticus. It may be that this 

 is an important olfactory nucleus, but I have been unable to 

 trace any fibers to it from the olfactory tract. It would be 

 impossible, however, to trace such fibers if they were present 

 in my preparations, owing to the enormous number of other 

 fibers among which they must run. There is some evidence 

 that this is an olfactory nucleus in the fact that some bundles 

 of the tractus olfacto-habenularis arise from this region, prob- 

 ably from some of the cells of the nucleus thaeniae here 

 described. 



c. THE FIBER TRACTS. - - Under this head I collect the 

 statements scattered through the preceding pages and make 

 some additions to them. 



(i) The tractus olfactorius is for the most part very diffuse. 

 The great majority of its fibers enter the fore-brain singly and 

 are at once mingled with the fibers of other tracts. Only a 

 small part of its fibers are gathered into a compact bundle. 



