Hhenomena of neurobiotaxis in the optic svstem 



273 



This is as well proved by the ventral position of the \'I nucleus, on 

 the ventral optic refiex-tract, as by the fact that a part of the nucleus abdu- 

 cens in bony-fishes, has a somewhat more frontal position (fig. 6j, than in 

 sharks, which is a consequence of the frontal origin of these optic stimulí. 



In the Pleuronectidae the nucleus abducens has a still more frontal 

 position than in other bony-fishes, the development of the fronto- 

 ventral optic reflex-tracts, being still stronger in these animáis. 



nc. Deit. 



f. 1. p. 

 nc. VI 



pFig. 4. — Dorsal position of the abducens nucleus next to the f. 1. p. in the 



Alligator, after v. Hoevell. 



Thiis we find in different representatives oj fishes, a different position 

 \of the VI nucleus, which may be explained by the stronger development oj 

 \one of the chief reflex-tracts for the eye movements. 



For comparative investigations, made in this regard on an more ex- 

 ttensive material, I specially refer to van der Horts elabórate dissertation 

 ¡and also to Davidson Blacks recently published researches. 



As a rule in all higher animáis the position of the nucleus abducens 

 [is a dorsal one. t 



Among Amphibia this is not so evident, the fase. long. posterior 

 ¡being separated from the floor of the 4th ventricle by a rather conside- 



11 orne naje a Caja I iS 



