VIII.
MORPHOGENESIS
OF THE VERTEBRATE
EYE
349
Bombina bombina in 40-5%, in Ambystoma mexicanum in 29-1%; in
R. temporaria the lenses appear in 100% of cases even at this stage.
Thus, species differences of the ectoderm are not of a qualitative but
of a quantitative-temporal character. In accordance with this, in those
species in which the capacity to develop Tree' lenses in the absence of
eyes appears earlier, the ability of the belly ectoderm to form lenses
under the action of the eye also disappears earlier. Discrepancies in the
data of various authors (Spemann, 1912; Ubish, 1927; Filatov, 1925;
Woerdeman, 1939; Popoff et al., 1938; review in Reyer, 1954b) in
obtaining free lenses in Rana esculenta after removal of eye rudiments
in the neurula turned out to be related to the temperature at which the
embryos were kept prior to operation. At low temperatures ( + 10°) the
rate of morphological change in embryos lags behind the rate of alteration of ectodermal potencies, so that, for example, the ectoderm of an
embryo which appears to be an early neurula has really reached a later
stage. It is in such R. esculenta embryos that free lenses arise, while this
does not happen in embryos kept at a higher temperature (+ 25°) (ten
Cate, 1953). Temperature dependence is found in such an extreme form
as the newt Taricha torosa where poorly developed free lenses were
obtained in a considerable percentage in this way (Jacobson, 1958).
But why do lenses arise without the action of the eye? It would be
natural to expect that their origin depends on other influences preceding
contact of the eye with the ectoderm, when the ectoderm is in all species
still sufficiently young to be affected by these influences. Some series of
differently conclusive experiments show that such influences begin with
the head entomesoderm coming into contact with the ectoderm during
gastrulation. Combining prechordal mesoderm of the middle gastrula
with belly ectoderm of the late gastrula of Triturus pyrrhogaster and
Rana japónica, Iz. Kawakawi (1952) obtained the induction of lenses
and nasal placodes. He thus proved that the prechordal entomesoderm
is the part which gives the first stimuli to lens development. This found
support in the experiments of Mangold (1954—Triturus alpestris) who
isolated head ectoderm at the neurula stage with the underlying entomesoderm, and Jacobson (1955, 1958) who isolated it both with the
whole entomesoderm, and separately with the entoderm and mesoderm
lying caudally to the place of lens appearance, in Taricha torosa. In all
variants small lenses or lentoids appeared in some cases. When the
ectoderm alone was isolated at this stage no lens arose in any species
(Perry, 1934; Woerdeman, 1939; Jacobson, 1958; Brahma, 1959) with
the exception of Ambystoma punctatum where lentoids appeared
(Liedke, 1955).
In another type of experiment head and belly ectoderm at a number of
stages were transplanted to the head, overlying the eye rudiment of
MORPHOGENESIS
OF THE VERTEBRATE
EYE
349
Bombina bombina in 40-5%, in Ambystoma mexicanum in 29-1%; in
R. temporaria the lenses appear in 100% of cases even at this stage.
Thus, species differences of the ectoderm are not of a qualitative but
of a quantitative-temporal character. In accordance with this, in those
species in which the capacity to develop Tree' lenses in the absence of
eyes appears earlier, the ability of the belly ectoderm to form lenses
under the action of the eye also disappears earlier. Discrepancies in the
data of various authors (Spemann, 1912; Ubish, 1927; Filatov, 1925;
Woerdeman, 1939; Popoff et al., 1938; review in Reyer, 1954b) in
obtaining free lenses in Rana esculenta after removal of eye rudiments
in the neurula turned out to be related to the temperature at which the
embryos were kept prior to operation. At low temperatures ( + 10°) the
rate of morphological change in embryos lags behind the rate of alteration of ectodermal potencies, so that, for example, the ectoderm of an
embryo which appears to be an early neurula has really reached a later
stage. It is in such R. esculenta embryos that free lenses arise, while this
does not happen in embryos kept at a higher temperature (+ 25°) (ten
Cate, 1953). Temperature dependence is found in such an extreme form
as the newt Taricha torosa where poorly developed free lenses were
obtained in a considerable percentage in this way (Jacobson, 1958).
But why do lenses arise without the action of the eye? It would be
natural to expect that their origin depends on other influences preceding
contact of the eye with the ectoderm, when the ectoderm is in all species
still sufficiently young to be affected by these influences. Some series of
differently conclusive experiments show that such influences begin with
the head entomesoderm coming into contact with the ectoderm during
gastrulation. Combining prechordal mesoderm of the middle gastrula
with belly ectoderm of the late gastrula of Triturus pyrrhogaster and
Rana japónica, Iz. Kawakawi (1952) obtained the induction of lenses
and nasal placodes. He thus proved that the prechordal entomesoderm
is the part which gives the first stimuli to lens development. This found
support in the experiments of Mangold (1954—Triturus alpestris) who
isolated head ectoderm at the neurula stage with the underlying entomesoderm, and Jacobson (1955, 1958) who isolated it both with the
whole entomesoderm, and separately with the entoderm and mesoderm
lying caudally to the place of lens appearance, in Taricha torosa. In all
variants small lenses or lentoids appeared in some cases. When the
ectoderm alone was isolated at this stage no lens arose in any species
(Perry, 1934; Woerdeman, 1939; Jacobson, 1958; Brahma, 1959) with
the exception of Ambystoma punctatum where lentoids appeared
(Liedke, 1955).
In another type of experiment head and belly ectoderm at a number of
stages were transplanted to the head, overlying the eye rudiment of