300 



C. U. Ariijns Kappers 



olf. epith 



optic stalk 



tal protrusion on both sides towards these stimuli. These paired, frontal 

 protrusions of neurobiotactic origin, constitute the aniphirrinal rhincnce- 

 phalon.Úie íorm of wliich is determined by tlie paired olfactory placodes ^. 

 It is obvious that \ve have to do here with a process closely resem- 

 bling the formation of the eye-vesicle, and I cannot refrain from digres- 



sing somewhat on this analogy as well as on 

 some differences, so that \ve may better realise 

 the character of the formation of both vesicle- 

 like protrusions. 



As another point of analogy we have the 

 fact that in the organ of smell it is not the 

 primary neurone but a higher one that runs in 

 the so-called olfactory stalk, which especially in 

 lower animáis may be very long, much longer 

 than the optic stalk (fig. IJ). 



These olfactory stalks are no more «nerves» 

 than the eye-stalk or opticus, since they also 

 contain neurones of a higher range, whose ori- 

 ginal cells, together with smaller link-cells, 

 mostly are lying cióse to the olfactory placode, 

 a position resembling that of the original cells 



Fig- 27. — Right olfactory of the opticus and many link-cells in the retina, 

 and optic stalks in a bonv . . 



fish: Sihu-Lis glanis. ' that lie near the light percipient cells. 



A great difference however is established by 



the fact that, whereas in the eye, the neuroepithelium cells form a part of the 



protruding brainvesicle, the olfactory epithelium remains in the epidermis. 



The olfactory cells themselves, as well as the olfactory membrane as a whole, 

 often grow a little inward, though remaining in contact with the surrounding 

 médium. In fishes the paired smell placodes cven form goblet shaped organs 

 which only ofier a small opening sometimes bearing a likeness to the eye of Halio- 

 tis. (Cf. figs. 26 and 28.) 



Why the smell placode makes this goblet shaped protrusion is difficult to ex- 

 plain. Perhaps it is simply caused by the neuro-epithelium. which, being subject 

 to a stronger growth than seems to be compatible with its origina! placodal 



' This is proved also teratologically by the case described by Riese (Riechhirn- 

 mangel; Zeitschr. f. d. ges. Neurología and Psychiatrie Bnd. 69, 1921) where the 

 paired protrusion of the forebrain failet together with the absence of peripheral 

 olfactorv sense oraans. 



