Phenomena of neurobiotaxis in the optic system 307 



The phylogenetic and ontogenetic developement of the nuclei of the 

 eye-muscle nerves show considerable alterations in the position of these 

 nuclei which point to the fact that the position of their cells is determi- 

 ned by the chief stimuH proceeding to them. 



Similarly the progressive development of the external geniculate 

 nucleus and its preponderance in higher mammals is due to stimulative 

 relations, in casu the increasing importance of the fibers from the fundus 

 oculi. That from this nucleus and not from the tectum the large cortical 

 representation arises is probably due to a nearer correlation of the fundus 

 (later maculafibers) with stereognostic visión, stereognosis being repre- 

 sented also in the thalamus (not in the tectumj and being equally con- 

 nected with the cortex. 



The change in the optic tracts, being totally crossed in lower vertebra- 

 tes and acquiring a partial homolateral course in mammals, is due to 

 binocular visión as is rightly brought forward by ¿". Ramón y Cajal. 



In consequence of this binocular visión temporal fibers of both sides 

 are stimulated at the same time (in frontal visión) and consequently partly 

 run together, keeping partly their course with nasal fibers of the other 

 side with which they are in stimulative correlation in lateral visión. 



The partial homolateral change of temporal fibers may be also respon- 

 sible tor the bilateral representation of the macula on the cortex. 



The formation of the vertébrate eye vesicle is to be considered from 

 same stand-point as the formation of the olfactory vesicle, i. e. it is caused 

 by external stimulative influences or their mnemic representives. These 

 stimulative influences also cause the hollow form of the double-walled 

 monoapertural eye, wliich is influenced by the divergence of the light in 

 monoapertural light perception. Similarly the convex eye of insects finds 

 its origin in the pluriapertural light perception in these animáis and the 

 convergent form of their field of visión. 



