Kiinctional factor.s in the m()ri)holo<^y of the forchrain of fishes 159 



physiological factor chiefly responsible for the difíerentiation ot the 

 lateral lobes. 



The lateral lobes of the telencephalon of Acanthias are occupied 

 exclusively by the área olfactoria, and a small part of this área is extended 

 backward into the telencephalon médium. The área olfactoria includes 

 the same three subdivisions which have already been described for Aci- 

 penser and Petromyzon (área olfactoria medialis,dorsalis and lateralis; and 

 in addition a ventral subdivisión, the tuberculum olfactorium. Behind the 

 área olfactoria lateralis is an área somática in relations similar to those of 



Fig. 12. — Diagram of the relations of the walls of the forebrain and its ventri- 

 cles in the dogfish, Acanthias. The relations of these ventricles are quite diffe- 

 rent from those of Mustelus canis as figured by Johnston (1906, fig. 8), but they 

 conform closely to his diagram (1906, fig. 9) «to show what is believed to be the 

 primitive relations of the wall of the ventricle». 



Acipenser. The distribution of the fibers of the tractos olfactorius to 

 the área olfactoria as seen from the lateral aspect is shown in figure 14. 

 The connections of some of the other fiber tracts are diagrammatically 

 indicated in figures I 5 and ló. Figure 17 illustrates the median section 

 of this brain. 



We shall now pass in review briefly the chief characteristics of these 

 subdivisions of the telencephalon of Acanthias, basing the description 

 largely on the studies of Johnston (1911). 



I. The área olfactoria medialis is very much larger than in most 

 other fishes. It has pushed far forward to form the rostral end of the 

 lateral lobe (figs. 14, 18) and in the mid-plane it unites with the opposite 

 nucleus to form the ventral part of the massive septum between the lat- 

 eral ventricles (fig. 17). This thickening of the lamina terminalis extends 



