SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 223 



yolk-sac, as they swim about is doubtless regulated by law with re- 

 gard to light. Most probably the pheuomenon usually called 'Higlit' 

 should be classed araoug ' tropisms.' It is present at a time at which 

 a developed nervous system is out of the question." Again, p. 285, 

 "A peculiarity present in even the youngest brood of fish is a recoil 

 from sadden optical or other light impressions. This ' flight reflex,' as 

 we shall call it for the sake of brevity, is retained by all fishes beyond 

 the stage of maturity." 



This flight reflex is a striking and interesting phenomenon in many 

 fishes. I have studied it experimentally in many species of young 

 fishes in the aquaria of the U. S. Fish Commission at Wood's Hole, but 

 one illustration will suffice. A tank 6x4x4 feet contained some 50 or 

 60 young mackerel, 3 to 4 inches long, which had recently been taken. 

 They responded instantly and almost as a unit to the slightest move- 

 ment on the part of a person standing before the tank. If the extended 

 arm were moved horizontally to the right, the school of fish instantly and 

 in unison moved horizontally to the left. If the movement of the arm 

 were downward, the fish moved upward, or if an oblique movement 

 were made, the fish responded promptly by a movement obliquely in 

 the opposite direction. As the hand was withdrawn, the fish followed 

 it back, as if actuated by some force of attraction, but probably because, 

 the stimulus for flight being removed, they tend to disseminate them- 

 selves through the tank. 



All fishes show this reaction in a greater or less degree. Most fishes, 

 however, fail to respond after they have been kept in aquaria for some 

 time and have become accustomed to the movements of people about 

 the tanks. There is little question, I believe, but that this flight reflex 

 is a response to stimuli through the action of the optic reflex apparatus 

 described in this paper. 



As noted by Edinger, the reflex is present before the fibre-tracts of 

 the brain and cord are developed. This agrees with the conditions I 

 have found in many fishes, where the cells of the apparatus are the first 

 in the brain to become differentiated, and the optic reflex apparatus is 

 fully established at the time of hatching, while the fibre-tracts of the 

 brain are as yet undeveloped. 



This flight reflex is early found in the larvae of all teleosts and 

 amphibians. In the cyclostomes and ganoids, however, tlie apparatus 

 is not fully developed until some days after ho,tching. There are 

 published some data to show that during these days the larvae are 

 sluggish and fail to respond to optical stimuli. I have had no op- 



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