VIII.
MORPHOGENESIS
OF THE VERTEBRATE EYE
341
and transform into vesicles. In the cases with maximal distension these
vesicles are made of pigment epithelium only. However, the differentiation of the latter in such an experiment always requires the presence of
the mesenchyme. But even the mesenchyme brings about a differentiation of the pigment epithelium in the first place in those sites where the
walls of the eye rudiment are thin (or where a superficial cell layer is
secondarily separated under the action of the mesenchyme).
In Amphibians the mesenchyme does not cover the eye rudiment
where the eye sticks closely to the epidermis (in Mammals the mesenchyme undergoes secondary degeneration at this site). This is sufficient
for retinal differentiation here. But, apart from this, the act of adhesion
to the epidermis condenses the eye rudiment, contributing to its
thickening here (Lopashov, 1946, 1948, 1960). Such initial conditions for
the differentiation of the retina and pigment epithelium were shown also
for Acipenserid fishes (Dabaghian, 1958, 1959) and for Mammals
(Stroeva, 1956a, 1960). The data of Dorris (1938), Gayer (1942), and
Winnikov (1947) suggest that a similar mechanism is likely to exist in
Birds as well.
When the eye vesicle was explanted with various amounts of mesenchyme, or transplanted to various parts of the amphibian emyryo,
there was noticed a relation between the amount of surrounding mesenchyme and the degree of differentiation of the pigment epithelium
(Lopashov, 1946, 1948, 1960). The action of mesenchyme in making the
pigment epithelium become thinner may consist both in its participation
in the formation of the basal membrane which fixes the epithelial state
of the tissue (Lopashov, 1960), and in its inhibition of mitoses (Takaya,
1956). The action of the mesenchyme seems to be also of a metabolic
character, since the pigment epithelium can arise without its action
when direct contact of the eye anläge with blood vessels occurs both in
Amphibians (Lopashov, 1948, 1951, 1960), and in Birds (Neyfakh, 1951).
In Urodeles (Lopashov, 1960), and in Acipenserid fishes (Dabaghian,
1958), the effect of the mesenchyme suffices only for the onset of the
first phases of pigmentation. Complete blackening is achieved only after
blood enters the choroid coat of the eye. When cultivating whole eye
rudiments of chicks, only that part of their external layer which is in
contact with air becomes pigmented. This shows the important role of
oxidative processes in the differentiation of the pigment epithelium
(Reinbold, 1954).
Simultaneously with the onset of pigmentation of the external eye
layer pigment granules in the retina form and their subsequent release
into the primary cavity of the eye follows. Spemann (1912) was the first
to observe this. These phenomena are clearly seen in eyes, where the
retina and pigment epithelium do not adjoin, and in accessory cavities
MORPHOGENESIS
OF THE VERTEBRATE EYE
341
and transform into vesicles. In the cases with maximal distension these
vesicles are made of pigment epithelium only. However, the differentiation of the latter in such an experiment always requires the presence of
the mesenchyme. But even the mesenchyme brings about a differentiation of the pigment epithelium in the first place in those sites where the
walls of the eye rudiment are thin (or where a superficial cell layer is
secondarily separated under the action of the mesenchyme).
In Amphibians the mesenchyme does not cover the eye rudiment
where the eye sticks closely to the epidermis (in Mammals the mesenchyme undergoes secondary degeneration at this site). This is sufficient
for retinal differentiation here. But, apart from this, the act of adhesion
to the epidermis condenses the eye rudiment, contributing to its
thickening here (Lopashov, 1946, 1948, 1960). Such initial conditions for
the differentiation of the retina and pigment epithelium were shown also
for Acipenserid fishes (Dabaghian, 1958, 1959) and for Mammals
(Stroeva, 1956a, 1960). The data of Dorris (1938), Gayer (1942), and
Winnikov (1947) suggest that a similar mechanism is likely to exist in
Birds as well.
When the eye vesicle was explanted with various amounts of mesenchyme, or transplanted to various parts of the amphibian emyryo,
there was noticed a relation between the amount of surrounding mesenchyme and the degree of differentiation of the pigment epithelium
(Lopashov, 1946, 1948, 1960). The action of mesenchyme in making the
pigment epithelium become thinner may consist both in its participation
in the formation of the basal membrane which fixes the epithelial state
of the tissue (Lopashov, 1960), and in its inhibition of mitoses (Takaya,
1956). The action of the mesenchyme seems to be also of a metabolic
character, since the pigment epithelium can arise without its action
when direct contact of the eye anläge with blood vessels occurs both in
Amphibians (Lopashov, 1948, 1951, 1960), and in Birds (Neyfakh, 1951).
In Urodeles (Lopashov, 1960), and in Acipenserid fishes (Dabaghian,
1958), the effect of the mesenchyme suffices only for the onset of the
first phases of pigmentation. Complete blackening is achieved only after
blood enters the choroid coat of the eye. When cultivating whole eye
rudiments of chicks, only that part of their external layer which is in
contact with air becomes pigmented. This shows the important role of
oxidative processes in the differentiation of the pigment epithelium
(Reinbold, 1954).
Simultaneously with the onset of pigmentation of the external eye
layer pigment granules in the retina form and their subsequent release
into the primary cavity of the eye follows. Spemann (1912) was the first
to observe this. These phenomena are clearly seen in eyes, where the
retina and pigment epithelium do not adjoin, and in accessory cavities