234 LESLIE B. AREY 



'I'hc (>lYo('t of se\'(M'iiif2; the optic norvo of Fiin(hilus was investi- 

 giited in a simihir nuimior. When brought from darkness to 

 light the pigment migrated normally after the usual operation. 

 The converse experiment also showed an independent movement 

 of the pigment, although the poorly defined contraction which 

 characterizes dark adaption rendered this type of experiment 

 less decisive than the clear cut results on Abramis. 



It is evident, then, that unlike Ameiurus the movements of 

 the retinal elements of Abramis and Fundulus are not dependent 

 upon the integrity of the optic nerve. From the results of Ham- 

 burger ('89) and Fick ('91) upon the frog, who showed that the 

 retinal pigment underwent movements after the optic nerve was 

 cut at the chiasma, it is safe to conclude that in this animal also 

 the optic nerve does not control pigment migration. The experi- 

 ments of these latter workers are all the more interesting since 

 in many observations which have been recorded showing the 

 retinal elements of the frog to be under nervous control, it has 

 generally been directly stated, or at least implied, that the optic 

 nerve was involved. It would be unprofitable to speculate con- 

 cerning the further occurrence among the vertebrates, or even 

 among the fishes, of individuals possessing efferent optic nerve 

 fibers which act similarly to those in Ameiurus. 



THEORETICAL CONSIDERATIONS 



Although serious doubt has beezi cast on many of the earlier 

 results upon the frog which were supposed to demonstrate the 

 presence of efferent fibers, both between the brain and the 

 retina and between the two retinas by way of the optic chiasma, 

 yet it is probable that there is a residue of truth in the general 

 proposition of a nervous control of the retinal elements in this 

 animal. 



Fick ('90, p. 84), in a paper attacking Englemann's assertion 

 of the existence of efferent nerve fibers, takes the following 

 position : 



Wenn dieser Schliiss richtig ist, so kann man niir ruhig durch die 

 ganze bisherige Sinnesphysiologie einen Strich machen und ihre Erfor- 

 schung von Neuem beginnen ; denn die rein centripetale und specifische 



