128 VERTEBRATE PHOTORECEPTORS 



ment and that vitamin A ordinarily associated with the 

 environment in which the fish is spawned. For example, the 

 anadromous white perch possesses porphyropsin only, yet 

 its close marine relative, the black sea bass, possesses only 

 rhodopsin. Likewise, the anadromous ale wife contains only 

 porphyropsin, but its close relative, the permanently marine 

 herring contains rhodopsin exclusively. The anadromous 

 brook trout, rainbow trout, and chinook salmon (all sal- 

 monids) were found to contain mixtures of rhodopsin and 

 porphyropsin — predominately the latter. In the catadro- 

 mous eel and killifish, however, which contain both pig- 

 ments, rhodopsin was found to predominate. It became 

 obvious from these findings that the pattern of retinal vita- 

 min A is not primarily an environmental response, but 

 is fixed genetically, and its significance is to be sought in the 

 phylogeny of fishes. 



Originally, having found vitamin A2 only in fresh water 

 and anadromous teleosts, Wald (19396) proposed that this 

 vitamin may represent a comparatively recent evolutionary 

 development, associated with the re-migration of marine 

 fishes into fresh water. More recently, however, Wald 

 (1942a, 19426) has found the porphyropsin system to pre- 

 dominate also in the anadromous lamprey, a cyclostome and 

 one of the most primitive of living vertebrates. He concluded 

 that the association of vitamin Ao with the capacity for 

 fresh water existence is, therefore, a very general and primi- 

 tive property, at least among vertebrates. 



In addition to these important findings regarding the 

 rod visual systems, Wald (19376) also partially isolated 

 from the chick a cone photopigment which he designated 

 iodopsin. This was accomplished by irradiating chicken 

 retinal extracts with deep red light, a method which has 

 long been known to stimulate the cones but not the rods 

 and rhodopsin. The initial bleaching which was obtained 

 by red light was believed to be due to the photopigment of 

 the cones. The bleaching properties showed it to be a violet 

 pigment (hence the name iodopsin), with an absorption 



