338
GEORGE WALD
yellow eels to possess various proportions of vitamins Ai and A 2 in their
retinas, and mixtures of rhodopsin and porphyropsin with net A max 504513 m/x. The porphyropsin alone could be shown by differential bleaching in the presence of hydroxylamine to have A max 523 m/x; the rhodopsin, by regeneration with neo-b retinene was shown to have A max 501502 m/x. As in the American species, in the average eel rhodopsin had a
higher extinction than porphyropsin, but was present in a slightly lower
molar concentration; a little more vitamin A 2 was present in the retina
than Ai.
The silver eels however had entirely lost their vitamin A 2 , and possessed vitamin A x alone in the retinas. This change seems to antedate
the transition from ordinary to deep-sea opsin, for in various silver eels
that had already lost their vitamin A 2 the A max of the visual pigment
ranged between 500 m/x and 487 m/x, as though the opsin were in various
stages of transition.
X. Invertebrates
Many of the lower invertebrates possess phototropic and phototactic
reactions, and in a number of cases the action spectra associated with
these have been determined. I know of no instance as yet, however, in
which a reliable study of the spectral sensitivity has been coupled with
the examination of the pigments involved. It is high time that such studies were undertaken and pursued systematically, for this is a field that
offers abundant material and every promise of revealing new relationships.
It is only in the mollusks and arthropods that well-developed imageforming eyes are encountered. The microstructure of the photoreceptor
organelles has lately been examined in representatives of both phyla,
and found to present the same basic pattern in all. Whereas in vertebrate eyes the rod and cone outer segments are built of transverse
layers (62), the fundamental structure of the rhabdome, the comparable
organ in invertebrate eyes, consists of transverse parallel tubules, so
that in longitudinal section, depending on the orientation, one sees
either an appearance of layers because one is cutting along the tubules,
or a honeycomb structure because one has cut across their ends. This
arrangement has been shown in the rhabdomeres of the compound
eyes of insects (63), in the arachnid Limulus, and a variety of Crustacea
(64), and in the so-called rods of the squid (65).
The visual systems of few arthropods and mollusks have yet been
analyzed chemically, but all of them so far examined are based upon
vitamin Αχ and retinene! alone. It should be noted at once that this
basic similarity in composition to the systems of vertebrate vision does
GEORGE WALD
yellow eels to possess various proportions of vitamins Ai and A 2 in their
retinas, and mixtures of rhodopsin and porphyropsin with net A max 504513 m/x. The porphyropsin alone could be shown by differential bleaching in the presence of hydroxylamine to have A max 523 m/x; the rhodopsin, by regeneration with neo-b retinene was shown to have A max 501502 m/x. As in the American species, in the average eel rhodopsin had a
higher extinction than porphyropsin, but was present in a slightly lower
molar concentration; a little more vitamin A 2 was present in the retina
than Ai.
The silver eels however had entirely lost their vitamin A 2 , and possessed vitamin A x alone in the retinas. This change seems to antedate
the transition from ordinary to deep-sea opsin, for in various silver eels
that had already lost their vitamin A 2 the A max of the visual pigment
ranged between 500 m/x and 487 m/x, as though the opsin were in various
stages of transition.
X. Invertebrates
Many of the lower invertebrates possess phototropic and phototactic
reactions, and in a number of cases the action spectra associated with
these have been determined. I know of no instance as yet, however, in
which a reliable study of the spectral sensitivity has been coupled with
the examination of the pigments involved. It is high time that such studies were undertaken and pursued systematically, for this is a field that
offers abundant material and every promise of revealing new relationships.
It is only in the mollusks and arthropods that well-developed imageforming eyes are encountered. The microstructure of the photoreceptor
organelles has lately been examined in representatives of both phyla,
and found to present the same basic pattern in all. Whereas in vertebrate eyes the rod and cone outer segments are built of transverse
layers (62), the fundamental structure of the rhabdome, the comparable
organ in invertebrate eyes, consists of transverse parallel tubules, so
that in longitudinal section, depending on the orientation, one sees
either an appearance of layers because one is cutting along the tubules,
or a honeycomb structure because one has cut across their ends. This
arrangement has been shown in the rhabdomeres of the compound
eyes of insects (63), in the arachnid Limulus, and a variety of Crustacea
(64), and in the so-called rods of the squid (65).
The visual systems of few arthropods and mollusks have yet been
analyzed chemically, but all of them so far examined are based upon
vitamin Αχ and retinene! alone. It should be noted at once that this
basic similarity in composition to the systems of vertebrate vision does