Phenomcna of neurobiotaxis in the oi)tic system 303 



That the lense is of no account for the hollow form of the eye, is preved bv the 

 lenseless, yet hollow eyes of some molluscs as Haliotis (fig. 28; and Xautilus. 



Resides the fact that in many vertebrates.the lense has dwindled to a size which 

 rnakes its contact with the neuroepitheliiim impossible in whatever stage of deve- 

 lopment, is another argument against the oi>inion that the hollow form of the eye 

 and the concrescence of the neuroepithelial and pigment layers wei"e due to a 

 pressiire of the lense. That merely an increase of the intraocular pression or an 

 enlaigement of the corpus vitreum should be the cause, is equally improbable. 



The junction of the pigment and neuroepithehal laj^ers is a functional 

 one and shows changes, according to the exposure to hght and due to 

 tropisms. 



Though I am far away from pretending to be able to solve the pro- 

 blem of the primitive junction of the layers, I am inclined to believe that 

 hght tropistic influences may be of primary iniportance here. 



In the meeting of the two walls the photopetal character of the pig- 

 ment together with the photoconcurrent tropism of the neuroepithelium 

 might act the chief part, these motions being opposite eachother. 



In the eyes of some Reptiles (Beíwi/er) only cones occur. In some other eyes 

 concs are prodominating and as the tropism of cones ^at least of their myoid) 

 bears a photo petal character (they shorten), this explanation must be wrong 

 unless their tropism in an early stage has been also a photofugal one, which indeed 

 may have been the case even if the supposition of some investigators (Parsons, 

 Grahatn Kerr) who consider the cones as developed from rods is wrong '. 



Detxeiler and Laurens or Cajal and Fürst, believe that both are derived from a 

 primitive neutral form. As all primitive placode cells in an early stage of their 

 evolution these may have had a stimulo-concurrent (photofugal) tropism as the 

 rods still have. 



Another form of tropism, may contribute to the concave torm of the 

 retina. I refer to the influencc of h"ght as determining the position of the 

 neuroepithehum, not its moving, but its orientating, directing influence. 

 Supposed that at first in phylogenetically primitive animáis, a considera- 

 ble part ol the skin had been of such thinness and transparency as to 

 permit the Hght, to the eyeviside and suppose this tranparent part to have 

 diminished by the further increase or pigmentation or thickness of the 

 skin, then the condition would be risen for the formation of a micro- 

 apertural lenseless camera-eye similar (non equal) as in Haliotis (fig. 2'$>) 



' Also the opposite opinión is held [Beniard and Cameron), that cones should 

 be the more primitive form (quoted after Deiwiler and Laureus. 



