PARKER: RETINAL PIGMENT CELLS OF PALZEMONETES. 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 exeised 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. 'l'here is no good evidence in favor of normal double 
conduction of nervous impulses. 
Nor, 
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 Verän- 
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 Fortsätzen der Irispigmentzellen [— distal retinular cells] 
aufgenommen, die, wie wir gesehen haben, mit dem im Vorderende der 
Retinulae angesammelten Pigmente im Contact stehen. An diesen 
Fortsätzen 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 
Ausläufern versehenen Zellen aufgenommen wird.” This idea that the 
