Functional factors in the morphology of the forebrain of fishes 157 



into the bulbus olfactorius. At the lower end they are in direct contact 

 respectively with epithaiamus, thalamus, and hypothalamus, and probably 

 their ¡nternal structure has been determined by these diencephalic con- 

 nections, though the details of their functional relations are very inade- 

 quately known. The arrangement oí these zones is as follows: 



Telencephalon Uiencephalon 



,' Área olfactoria dorsalis Habenula and lobus 



\ subhabenularis. 



Bulbus olf. Área olfactoria lateralis, lobus subhippocampalis, 



I «striatum> Thalamus. 



f Área olfactoria medialis, nucleus preopticus .... Hvpothalamus. 



Squalus acanthias. 



The brains of the sharks in some respects appear to resemble most 

 closely those of cyclostomes, but in other ways they are unique. The 

 olfactory bulbs are widely evaginated; but the remainder of the telence- 

 phalon exhibits a very peculiar type of invagination, a modicum of eva- 

 gination, and considerable local thickening, with in Acanthias a slight 

 tendency toward eversión of the dorsal border posteriorly. 



The general relations of the walls and cavities of the forebrain are 

 shown in figure 12. The common telencephalic ventricle (ventriculus 

 commune) divides in front into two lateral ventricles, from each of which 

 an olfactory ventricle extends laterally and forward. The bulbus olfac- 

 torius and its ventricle are similar to those of Acipenser (fig. I), but 

 much larger. The ventriculus lateralis, however, has a form which is 

 peculiar to elasmobranchs and is not exactly comparable with that of 

 either Petromyzon (fig. 7) or Amphibia (fig. 30), for it has been cut off 

 from the common telencephalic ventricle by a different process. 



The steps in the embryological development of the telencephalon of 

 Acanthias have been described and figured by Sterzi (I912, p. 1206; sea 

 figure 13). In early stages the rostral part of the ventriculus commune 

 dilates laterally and from the lateral bordcrs of this expanded portion the 

 olfactory bulbs are evaginated (stages from 32 mm. to 92 mm. long). 

 During the process of formation of the olfactory bulbs each lateral half 

 of the common ventricle evaginates laterally, dorsally, and forward, the 



