VIII.
MORPHOGENESIS OF THE VERTEBRATE EYE
351
respects. Under the action of the entomesoderm there arise not only
lenses but other ectodermal anterior head (archencephalic) structures,
nasal placodes in particular (Iz. Kawakami, 1952; Jacobson, 1958). In a
younger ectoderm, entomesoderm induces also neural tissue (ter Horst,
1948; Takaya, 1953a, b; Sala, 1955; Hoessels, 1957). In contrast, eye
rudiments act selectively: they do not induce neural tissue in gastrula
ectoderm (Schmidt and Ragosina, 1937; Lopashov, 1937; Woerdeman,
1938). Their affinity for ectoderm probably begins to develop only after
the ectoderm reaches a later stage; it is associated with the beginning
of the contact between the eye and the ectoderm and increase of the
close adhesion in the zone of their contact and retinal invagination.
The action of the eye in this zone is highly selective. At the same time,
nasal placodes can arise on contact of ectoderm with the posterior
surface of early eye rudiments and under the action of brain rudiments
separating from the former (Woerdeman, 1938; Ikeda, 1938; Lopashov,
1960). The action of the entomesoderm can be thought to be to
induce neither lens, nor nasal placode, but an incompletely segregated
complex of anterior cranial rudiments. In most species the action of the
entomesoderm leads to the formation of lentoids and lenses considerably
inferior in size to those of unoperated eyes. Subsequent action of the eye
(or that of the forebrain for nasal placodes—Haggis, 1956) leads to a
further increase of tendencies to the development of lenses and to the
distinction of the two tendencies, to eye or nasal placode formation.
The appearance of Tree lenses' in the ectoderm under the action of
various alien inductors (Toivonen, 1945; Iz. Kawakami, 1950; Becker,
1959) proves that the action of the entomesoderm and of the eye which
also lead to their appearance, is a transmission of some inducing agent.
This is also confirmed by the experiments of McKeehan (1958) who put
agar between the eye rudiment and the ectoderm in chicks. Such
experiments require, however, the employment of materials similar to
the membrane filters used by Grobstein (1955a) in experiments on other
tissues.
The two initiating phases of lens induction seem to represent a
mechanism for the increase and concentration of specific conditions for
lens formation in the ectoderm. Both actions are in the main synergistic
and the lens development depends on their combined action (if they act
in normal sequence). But if the second action, the effect of the eye,
works only a short time, then not only the effect of the entomesoderm,
but the effect of both of them turns out to be insufficient. Beginning
from Le Cron (1907), all work (Dragomirov, 1930; Manuilova, 1931;
Filatov, 1934; Woerdeman, 1939—on Amphibians, McKeehan, 1954—
on Birds) showed clearly that lenses formed after removal of the eyes
achieve a greater size, the longer the contact of their rudiment was
MORPHOGENESIS OF THE VERTEBRATE EYE
351
respects. Under the action of the entomesoderm there arise not only
lenses but other ectodermal anterior head (archencephalic) structures,
nasal placodes in particular (Iz. Kawakami, 1952; Jacobson, 1958). In a
younger ectoderm, entomesoderm induces also neural tissue (ter Horst,
1948; Takaya, 1953a, b; Sala, 1955; Hoessels, 1957). In contrast, eye
rudiments act selectively: they do not induce neural tissue in gastrula
ectoderm (Schmidt and Ragosina, 1937; Lopashov, 1937; Woerdeman,
1938). Their affinity for ectoderm probably begins to develop only after
the ectoderm reaches a later stage; it is associated with the beginning
of the contact between the eye and the ectoderm and increase of the
close adhesion in the zone of their contact and retinal invagination.
The action of the eye in this zone is highly selective. At the same time,
nasal placodes can arise on contact of ectoderm with the posterior
surface of early eye rudiments and under the action of brain rudiments
separating from the former (Woerdeman, 1938; Ikeda, 1938; Lopashov,
1960). The action of the entomesoderm can be thought to be to
induce neither lens, nor nasal placode, but an incompletely segregated
complex of anterior cranial rudiments. In most species the action of the
entomesoderm leads to the formation of lentoids and lenses considerably
inferior in size to those of unoperated eyes. Subsequent action of the eye
(or that of the forebrain for nasal placodes—Haggis, 1956) leads to a
further increase of tendencies to the development of lenses and to the
distinction of the two tendencies, to eye or nasal placode formation.
The appearance of Tree lenses' in the ectoderm under the action of
various alien inductors (Toivonen, 1945; Iz. Kawakami, 1950; Becker,
1959) proves that the action of the entomesoderm and of the eye which
also lead to their appearance, is a transmission of some inducing agent.
This is also confirmed by the experiments of McKeehan (1958) who put
agar between the eye rudiment and the ectoderm in chicks. Such
experiments require, however, the employment of materials similar to
the membrane filters used by Grobstein (1955a) in experiments on other
tissues.
The two initiating phases of lens induction seem to represent a
mechanism for the increase and concentration of specific conditions for
lens formation in the ectoderm. Both actions are in the main synergistic
and the lens development depends on their combined action (if they act
in normal sequence). But if the second action, the effect of the eye,
works only a short time, then not only the effect of the entomesoderm,
but the effect of both of them turns out to be insufficient. Beginning
from Le Cron (1907), all work (Dragomirov, 1930; Manuilova, 1931;
Filatov, 1934; Woerdeman, 1939—on Amphibians, McKeehan, 1954—
on Birds) showed clearly that lenses formed after removal of the eyes
achieve a greater size, the longer the contact of their rudiment was