VIII.
MORPHOGENESIS OF THE VERTEBRATE EYE
343
provides for its rolling up into a cup. The margins of the eye cup in the
explants completely close around the lens. In normal development,
however, the adhesion of the lens to the surface ectoderm prevents this
excessive rolling up (Lopashov, 1960). Thus the temporary adhesion of
the eye rudiment to the developing lens, and of the lens to the ectoderm,
is also necessary for the achievement of the appropriate extent of
invagination and for the shape of the eye cup.
Mechanical tension in the internal layer during the process of rolling
up into the eye cup also plays a role in Mammals. If an aggregation of
eye vesicle cells is a prerequisite for the formation of the retinal rudiment, it is tension in this layer which is required for its further growth
and differentiation. When eye vesicles of rats are cultivated without
mesenchyme and lens, in the anterior chamber of the adult eye, they
are transformed into retina which does not undergo further development. But when the retina is cultivated with the ectoderm of the lens,
it rolls up into an eye cup and is stretched by the growing lens; and it
then differentiates normally into layers (Stroeva, 1960). This feature
distinguishes Mammals from Amphibians (Lopashov, 1960) and from
Acipenserid fishes (Dabaghian, 1958), the retina of which differentiates
further in an aggregation, without forming a cup.
During the stages under analysis three axes of the retina are established. In order to reveal the time of determination of the choroid
fissure, Sato (1933) turned the eye vesicle or eye cup (stages 21-32) of
Triturus taeniatus round the proximal axis through 180°, with the overlying ectoderm. Supplemented by experiments at the early neurula stage
(Woerdeman, 1934), these investigations showed that the tendencies to
invagination and growth necessary for the choroid fissure to be formed
were realized, at all the stages investigated, independently of their surrounding. The appearance of a choroid fissure in these experiments on
the ventral side also, i.e. position-dependent, shows that dorsal areas of
the eye rudiment are still able to form a choroid fissure, but that the
surroundings also participate in its formation. Sato thinks that the main
role is played here by the developing ventral connection with the brain.
The frequency of a position-dependent development of the choroid
fissure decreases with age. The validity of the conclusions from these
experiments is however diminished by the fact that the eye rudiment
was not separated from the ectoderm, which might also exert a polarizing action upon it.
In experiments in which the eye rudiment of Triturus taeniatus was
turned through 90° or 180°, or in which two primary anterior or two
primary posterior halves of the eye rudiment where joined, and the
visual reflexes in the operated larvae where checked by the method of
Sperry (1944, 1945) and Stone Í1948, 1953), it was shown by Szekely
MORPHOGENESIS OF THE VERTEBRATE EYE
343
provides for its rolling up into a cup. The margins of the eye cup in the
explants completely close around the lens. In normal development,
however, the adhesion of the lens to the surface ectoderm prevents this
excessive rolling up (Lopashov, 1960). Thus the temporary adhesion of
the eye rudiment to the developing lens, and of the lens to the ectoderm,
is also necessary for the achievement of the appropriate extent of
invagination and for the shape of the eye cup.
Mechanical tension in the internal layer during the process of rolling
up into the eye cup also plays a role in Mammals. If an aggregation of
eye vesicle cells is a prerequisite for the formation of the retinal rudiment, it is tension in this layer which is required for its further growth
and differentiation. When eye vesicles of rats are cultivated without
mesenchyme and lens, in the anterior chamber of the adult eye, they
are transformed into retina which does not undergo further development. But when the retina is cultivated with the ectoderm of the lens,
it rolls up into an eye cup and is stretched by the growing lens; and it
then differentiates normally into layers (Stroeva, 1960). This feature
distinguishes Mammals from Amphibians (Lopashov, 1960) and from
Acipenserid fishes (Dabaghian, 1958), the retina of which differentiates
further in an aggregation, without forming a cup.
During the stages under analysis three axes of the retina are established. In order to reveal the time of determination of the choroid
fissure, Sato (1933) turned the eye vesicle or eye cup (stages 21-32) of
Triturus taeniatus round the proximal axis through 180°, with the overlying ectoderm. Supplemented by experiments at the early neurula stage
(Woerdeman, 1934), these investigations showed that the tendencies to
invagination and growth necessary for the choroid fissure to be formed
were realized, at all the stages investigated, independently of their surrounding. The appearance of a choroid fissure in these experiments on
the ventral side also, i.e. position-dependent, shows that dorsal areas of
the eye rudiment are still able to form a choroid fissure, but that the
surroundings also participate in its formation. Sato thinks that the main
role is played here by the developing ventral connection with the brain.
The frequency of a position-dependent development of the choroid
fissure decreases with age. The validity of the conclusions from these
experiments is however diminished by the fact that the eye rudiment
was not separated from the ectoderm, which might also exert a polarizing action upon it.
In experiments in which the eye rudiment of Triturus taeniatus was
turned through 90° or 180°, or in which two primary anterior or two
primary posterior halves of the eye rudiment where joined, and the
visual reflexes in the operated larvae where checked by the method of
Sperry (1944, 1945) and Stone Í1948, 1953), it was shown by Szekely