PAKKElt: KETINAL PIGMENT CELLS OF PAL^MONETES. 297 



13. The photomechanical action within the retina is localized, small 

 groups of pigment cells responding to local stimulation. 



14. In excised eyes (optic nerve cut), complete photomechanical 

 changes may occur, thus proving that the brain is not essential to these 

 changes. 



15. In excised retinas (retinal nerve fibres cut), nearly complete 

 photomechanical changes may occur, thus showing that the optic 

 ganglia are probably not essential to these changes. 



16. The incompleteness of the changes in either the excised eyes or 

 excised retinas is probably due to the death of the retinal tissues before 

 the photomechanical changes have been completed. 



17. The three kinds of retinal pigment cells probably respond to 

 direct stimulation from without, and are not influenced by nervous im- 

 pulses from within. There is no good evidence in favor of normal double 

 conduction of nervous impulses. 



Note. 



Since the preceding pages were written, Rosenstadt's ('96) paper on 

 the structure of the compound eyes in Decapods has been published. 

 This contains a brief general account of the migration of the retinal 

 pigment in these crustaceans, and calls for a word of comment. In his 

 description of the directions of motion shown by the pigment under 

 various conditions of light, Rosenstadt agrees with Exner and later 

 investigators, but in his account of how this movement is accomplished 

 he stands entirely alone. His conception of the process can best be put 

 in his own words (Rosenstadt, '96, p. 759) : " Beim Uebergange des 

 Lichtauges in ein Dunkelauge gehen mit dem Pigmente volgende Veran- 

 derungen vor sich : Das Pigment tritt aus dem vorderen Ende der 

 Retinulazellen [= proximal retinular cells] und wohl auch aus den 

 Retinapigmentzellen [= rudimentary retinular cells] aus. Dasselbe 

 wird von den Fortsatzen der Irispigmentzellen [= distal retinular cells] 

 aufgenommen, die, wie wir gesehen haben, mit dem im Vorderende der 

 Retinulae angesammelten Pigmente im Contact stehen. An diesen 

 Fortsatzen kriecht nun das Pigment hinauf; es findet eine Art Pig- 

 mentinfiltration der Irispigmentzellen statt. Gleichzeitig wandert das 

 Pigment nach hinten zu aus den Retinulazellen aus und gelangt hinter 

 die Membrana fenestrata [= basement membrane], wo es von den mit 

 Auslaufern versehenen Zelleu aufgenommen wird." This idea that the 



