302 C. U. Ariens Kappers 



The diretion (course) of rays is the most prominent factor in the direction 

 of organisms and it is possible by directed üght (not diffuse) to draw ani- 

 máis from a more, but ditTusely Hghted spot, to a less illumined one, 

 although by their positively phototropical character, one might expect 

 them to retain the place of greater light intensity ^ 



An instance of this influence seems to be given also by the eye vvhose 

 üght-percepting elements are grosso viodo orientated by the direction 

 ot the rays that reach them. 



As a first stage in the eye likewise as in the olfactory lobes we find 

 the outgrowth of a brainvesicle, in which however the young light-placo- 

 des are already enclosed. 



The outgrowth of the eyevesicle to the bodywall must be due to 

 tropistic factors from the periphery or their rnnemic recapitulations. 



In further development, we find that 

 the neuroepithelian layer, draws nearer to 

 the background of the eye. The eye- 

 vesicle, though more or less hollow in the 

 beginning, is not to be considered as 

 a hollow ball, which later is squeezed 

 mechanically into the shape of a double 

 cup. Especially Froriep in his excellent 

 treatnient on the development of the 

 eye, showed this representation to be 

 erroneous. Already in their first out- 

 ward protrusion, the two sides of the eye- 

 pjg 28. Lenseless vet concave vesicle are lying rather cióse to eachother, 



eye Avith small aperture in a mol- without coming into contact however. 

 lusc: Haliotis (the eye resembles 



that of Xautilus). Later a concrescence of the neuroepithe- 



lium and pigmental layer takes place, 



and írom this moment, the tlouble-walled «anlage» will grow to such an 



extent, that the outer rims approach eachother, so that the then double 



walled organ, again acquires the form of a hollow globe, showing only a 



small opening. It is needless to contradict the antiquated opinión that the 



junction of the neuroepithelium and the pigment-layer, as well as the 



rounding of the eye were due to the lense. 



' Loeb: «Vorlesungen über die Dynamik der Lebenserscheinungen», Bailh, 

 Leijjzig, 1906. 



