VIII. MORPHOGENESIS OF THE VERTEBRATE EYE
345
in the zone of contact with the ectoderm. This contact excludes the
opposing action of the mesenchyme; the mesenchyme action contributes
to the thinning of the other layer both directly and through the inhibition of mitoses, and this pathway of differentiation is strengthened later
by the blood supply. In the aggregation of cells giving rise to the retina,
on the contrary, there develop conditions which favour its further
growth by mitosis. The induction of the lens, which is also directly
related to prolonged contact between the eye rudiment and the ectoderm (§IV), contributes to increased aggregation of the retina and to its
rolling up into the cup. In its turn, this rolling up, stretching the
external layer, promotes its thinning and its development into the pigment epithelium. This latter, becoming ever thinner, improves conditions for the rolling up and growth of the retina. Thus, the phenomena
of opposing differentiations are inhibited in both layers, but, at the
same time, provided there are typical relations to other parts of the
embryo, the layers promote each other's development along their
specific pathways (Lopashov, 1960).
This segregation arises after the separation of the eye material which
takes place in the preceding phase (§11). Provided that this material
comes into contact with the mesenchyme and ectoderm in a sufficiently
restricted space, eyes of typical structure can then develop, independently, out of the preceding organization of the rudiment of the anterior
part of the neural plate. Such eyes arise in some cases when neural
structures are formed in pieces of ectoderm through sublethal cytolysis
(Holtfreter, 1944); they can also arise after the dissociation to separate
cells of the anterior part of the neural plate of the neurula (together with
the ectoderm) and the subsequent reaggregation of the cells (Boterenbrood, 1958).
After the onset of blood circulation, closure of the choroid fissure and
formation of closed mesenchyme envelopes, connections between the
ectoderm and the lens and between the lens and the internal surface of
the retina are broken, so that the further formation and growth of the
eye go on under the influence of a new complex mechanism. The role of
intraocular pressure in this mechanism is now well elucidated.
By cutting the eye wall (Weiss and Amprino, 1940) or introducing a
thin glass tube through which the vitreous body escapes into the
amniotic cavity (Coulombre, 1956) an artificial decrease of the intraocular pressure was obtained in chick embryos. If this decrease is realized
during the period of the most intense growth of the eye, which takes
place after the closure of the choroid fissure (4th to 8th days of incubation), then microphthalmia develops. In this, the pigment epithelium
acquires a size proportional to the eye, while the retina acquiring a
relatively much larger size becomes arranged in folds. This different
345
in the zone of contact with the ectoderm. This contact excludes the
opposing action of the mesenchyme; the mesenchyme action contributes
to the thinning of the other layer both directly and through the inhibition of mitoses, and this pathway of differentiation is strengthened later
by the blood supply. In the aggregation of cells giving rise to the retina,
on the contrary, there develop conditions which favour its further
growth by mitosis. The induction of the lens, which is also directly
related to prolonged contact between the eye rudiment and the ectoderm (§IV), contributes to increased aggregation of the retina and to its
rolling up into the cup. In its turn, this rolling up, stretching the
external layer, promotes its thinning and its development into the pigment epithelium. This latter, becoming ever thinner, improves conditions for the rolling up and growth of the retina. Thus, the phenomena
of opposing differentiations are inhibited in both layers, but, at the
same time, provided there are typical relations to other parts of the
embryo, the layers promote each other's development along their
specific pathways (Lopashov, 1960).
This segregation arises after the separation of the eye material which
takes place in the preceding phase (§11). Provided that this material
comes into contact with the mesenchyme and ectoderm in a sufficiently
restricted space, eyes of typical structure can then develop, independently, out of the preceding organization of the rudiment of the anterior
part of the neural plate. Such eyes arise in some cases when neural
structures are formed in pieces of ectoderm through sublethal cytolysis
(Holtfreter, 1944); they can also arise after the dissociation to separate
cells of the anterior part of the neural plate of the neurula (together with
the ectoderm) and the subsequent reaggregation of the cells (Boterenbrood, 1958).
After the onset of blood circulation, closure of the choroid fissure and
formation of closed mesenchyme envelopes, connections between the
ectoderm and the lens and between the lens and the internal surface of
the retina are broken, so that the further formation and growth of the
eye go on under the influence of a new complex mechanism. The role of
intraocular pressure in this mechanism is now well elucidated.
By cutting the eye wall (Weiss and Amprino, 1940) or introducing a
thin glass tube through which the vitreous body escapes into the
amniotic cavity (Coulombre, 1956) an artificial decrease of the intraocular pressure was obtained in chick embryos. If this decrease is realized
during the period of the most intense growth of the eye, which takes
place after the closure of the choroid fissure (4th to 8th days of incubation), then microphthalmia develops. In this, the pigment epithelium
acquires a size proportional to the eye, while the retina acquiring a
relatively much larger size becomes arranged in folds. This different