Phenomena of ncurobiotaxis in thc optic system 301 



«anlage», is compelled to make either an outward or an ¡nward protrusion. Its 

 inward protrusion might than be explained by the already establishcd connection 

 vvith the olfactoi-y \-esicles of the brajn. It is also possible that the smell-stimulus. 

 whilst causing the axons of the olfactory cells to grow in a stimulo concurrent 

 direction, i. e. toward ihc brain, at the same time may have a similar influence 

 011 the cells of origin of these axons, although they exhibit a much Icss pronoun- 

 ced stimulo-fugal character than some other placode-cells as f. i. those of the fa- 

 cialis, the acusticus, and the glossopharyngeiis, which in a very early stage show 

 an inward growth. (Also young ncuroblasts, before they hav'e developed dcndrites 

 and Nissl bodies show a stimulo-concurrent tendency.) 



That the olfactory neuroepitheliuní remains at the surface instead of 

 its placode-cells being included in the brainwall, as is the case with 

 the neuroepithelium of the eye, is not surprising after al!, since the 

 stimulating, odoriferous matter is dissolved in liquid it could not come 

 into contact with the olfactory cells, if these were separated from it 

 by an epithelial membrane. An olfactory cell would be lost as sucli 

 ií it carne to ly beneath tlie skin, as it thus would be out of reach ot 

 its specific stimulus ^ and also out of reach of the shifting influence 

 thereof. 



On the contrary in the same circumstances the light-placode, if we 

 may give this ñame to the retina's neuroepithelium, will not lose its func- 

 tion as an organ of light-perception since light can easily penétrate 

 through the covering tissue. 



Now we know light to have a twofold influence on organisms: 1° a di- 

 recting and 2° an attractive (or a repulsivel influence. 



Difterent ñames have been given to these tropisms and one has 

 called phototaxis the tendency of an organism to direct it self accor- 

 ding the direction of the light, and photopathy - the tendency of an- 

 organism to move (independently from its attitude) towards a distinct 

 intensiíy o f light. 



' Moreover it is only the sensehair that protnidcs in the nasal cavity and this 

 is easily explained by the fact, discovered by Van der Stricht, that this sensehair 

 bears the centrosome, which as I pointed out in my book on comparative anatomy 

 of the brain-has a stimulopetal character (vol. i. page 6.) Sometimes indeed, a 

 smell-placode will sink into the deep, as was proved by IVoerdemans investigations 

 of lampreys, where the single smcll-placode, at thc appearance of the paired ones, 

 partly at least, is lost as such and becomes a parí of the hypophysis cerebri, 

 acquiring another function (internal secretion). 



2 Compare Holí and Lee: «American Journal of l'hysiology» (vol. iv, 1901). 



