VIII.
MORPHOGENESIS
OF THE VERTEBRATE EYE
363
neural crest. Reciprocal transplantations of eye rudiments between
various newt and Ambystoma species at the stage before ectomesenchyme migration, transplantation of neural folds above the eye rudiments, and transplantation of eye rudiments to embryos devoid of
neural folds, have showed that iris chromatophores in Amphibians take
their origin from the neural crest as well (Barden, 1942).
Do the remaining cells of the eye envelopes also originate from this
source? Explantation of optic vesicles in the head ectoderm of embryos
together with neural crest show that a typical mesenchyme envelope
arises here which surrounds the eye but is separated from it by a small
gap bridged across by cell-free outgrowths of the mesenchyme. Upon
adhesion of such explants to the glass and a subsequent decrease of
their internal space, the mesenchyme network assumes an almost
perfect resemblance to that in the head of a normal embryo. But no
vessels arise in the eye envelopes either in such explants, or in isolated
heads, or in embryos the blood rudiment of which was removed at the
neurula stage (Lopashov, 1951, 1956, 1960). It seems that only the
primary mesenchyme envelope arises from the ectomesenchyme; this
primary envelope exists till the onset of blood circulation and the
formation of vessels, without which further formation of the choroid
coat is disturbed. Unlike the vessels of the choroid coat, the hyaloid
vessels in Anurans grow into the cavity of the eye cup in the absence of
blood (Jolly, 1944; Lopashov, 1960). Thus, at least in Amphibians, the
choroid coat is formed in two phases: (a) formation of the ectomesenchyme primary envelope, (b) formation of the secondary eye envelope
which takes place with the entrance of blood and mesodermal vesselforming cells in the space between the cytoplasmic outgrowths of the
mesenchyme envelope.
Do these ectomesenchyme cells of the primary mesenchyme envelope
take their origin from the neural crest only? Migrating mesenchyme cells
emerge from amphibian optic vesicles explanted in salt solution.
Urodeles have a few of them, Anurans have more, so that these cells
regularly form networks upon rudiments explanted in ectodermal
vesicles. These cells are released very early and if they become arranged
on the eye rudiment surface, the external layer of the eye rudiment
develops into pigment epithelium (Lopashov, 1956, 1960). While these
data obtained on Amphibians only allow one to assume the participation
of the eye rudiment as a source of this or that portion of the cells of its
primary envelope, a careful analysis of the development of the eye in
human embryos showed that a considerable number of cells composing
the mesenchyme envelope of the eye come from the eye rudiment at the
optic vesicle stage (Bartelmez, 1954). The same mesenchyme migration
was observed when cultivating the eye rudiments of rats at analogous
MORPHOGENESIS
OF THE VERTEBRATE EYE
363
neural crest. Reciprocal transplantations of eye rudiments between
various newt and Ambystoma species at the stage before ectomesenchyme migration, transplantation of neural folds above the eye rudiments, and transplantation of eye rudiments to embryos devoid of
neural folds, have showed that iris chromatophores in Amphibians take
their origin from the neural crest as well (Barden, 1942).
Do the remaining cells of the eye envelopes also originate from this
source? Explantation of optic vesicles in the head ectoderm of embryos
together with neural crest show that a typical mesenchyme envelope
arises here which surrounds the eye but is separated from it by a small
gap bridged across by cell-free outgrowths of the mesenchyme. Upon
adhesion of such explants to the glass and a subsequent decrease of
their internal space, the mesenchyme network assumes an almost
perfect resemblance to that in the head of a normal embryo. But no
vessels arise in the eye envelopes either in such explants, or in isolated
heads, or in embryos the blood rudiment of which was removed at the
neurula stage (Lopashov, 1951, 1956, 1960). It seems that only the
primary mesenchyme envelope arises from the ectomesenchyme; this
primary envelope exists till the onset of blood circulation and the
formation of vessels, without which further formation of the choroid
coat is disturbed. Unlike the vessels of the choroid coat, the hyaloid
vessels in Anurans grow into the cavity of the eye cup in the absence of
blood (Jolly, 1944; Lopashov, 1960). Thus, at least in Amphibians, the
choroid coat is formed in two phases: (a) formation of the ectomesenchyme primary envelope, (b) formation of the secondary eye envelope
which takes place with the entrance of blood and mesodermal vesselforming cells in the space between the cytoplasmic outgrowths of the
mesenchyme envelope.
Do these ectomesenchyme cells of the primary mesenchyme envelope
take their origin from the neural crest only? Migrating mesenchyme cells
emerge from amphibian optic vesicles explanted in salt solution.
Urodeles have a few of them, Anurans have more, so that these cells
regularly form networks upon rudiments explanted in ectodermal
vesicles. These cells are released very early and if they become arranged
on the eye rudiment surface, the external layer of the eye rudiment
develops into pigment epithelium (Lopashov, 1956, 1960). While these
data obtained on Amphibians only allow one to assume the participation
of the eye rudiment as a source of this or that portion of the cells of its
primary envelope, a careful analysis of the development of the eye in
human embryos showed that a considerable number of cells composing
the mesenchyme envelope of the eye come from the eye rudiment at the
optic vesicle stage (Bartelmez, 1954). The same mesenchyme migration
was observed when cultivating the eye rudiments of rats at analogous