IX. DEVELOPMENT OF THE TELEOSTEAN EGG
395
ment only one embryo is always formed as in experiments carried out on
older germs. The absence of a symmetry reference at the stages operated
upon makes it impossible to say whether the embryo corresponds to
the primary or the secondary(?) marginal zone. The stages operated
upon are probably too advanced; what the periblast can still supply is
not sufficient to create a second marginal zone that can compete with
the primary one.
Under certain conditions, however, one can allow the periblastic
material to manifest the remainder of its 'determinative power' (Fig. 11).
If the dorsal (embryonic) half of a blástula or a young gastrula is excised,
the ventral (extra-embryonic) half which remains rebuilds a circular
germ in which the embryo appears in its normal place, that is to say in
the place of the pre-existing dorsal zone. But when cultivated in vitro
without periblast or yolk, this zone is incapable of differentiation
(Richard etal, 1956).
Instead of simply suppressing the dorsal half, one can put the ventral
half (grafted half) of another germ in its place (as already shown in an
experiment by Luther, 1937); the embryo appears in the grafted half,
that is to say where the previous dorsal zone was. If the same type of
experiment is done with two dorsal zones, there is no competition, and
two embryos appear face to face.
Two facts must be taken into consideration: regulation of the germ
and determinism of the embryo's localization. We know, from Luther's
work (1936), that the blástula is totipotent. In the two types of experiments performed at this stage a normal embryo is formed. (See also
experiments of fissuration of the blastoderm in Salmo salvelinus; Lutz
and Lutz-Ostertag, 1952.) This totipotency has disappeared in the
gastrula, and, in these experiments, instead of a total regulation, there
is only an orderly positioning of the marginal material which is present
(regulation of the morphogenetic movements).
But in all cases, and here is the important phenomenon, the embryonic
material takes up the position in which the normal embryo would have
appeared if the germ had not been operated upon. The zone corresponding to the underlying vitelline syncytium seems to have a directing
influence and, in the case of the blástula, an organizing influence also.
At this point, and at this stage, it may be thought that part of the
crescent in its broadest portion remains and, in the absence of competition, is sufficient to make a new marginal zone appear.
In favour of the organizing influence of the 'crescent' one can quote
the result of the chronological combination (Devillers, 1952; see also
Section IV, A, 4) of a morula blastodisc with the yolk of an old gastrula
(Fig. 19): epiboly proceeds normally, but the embryo hardly organizes
itself: at the stage of epiboly 1/2 it is no more than a badly shaped
395
ment only one embryo is always formed as in experiments carried out on
older germs. The absence of a symmetry reference at the stages operated
upon makes it impossible to say whether the embryo corresponds to
the primary or the secondary(?) marginal zone. The stages operated
upon are probably too advanced; what the periblast can still supply is
not sufficient to create a second marginal zone that can compete with
the primary one.
Under certain conditions, however, one can allow the periblastic
material to manifest the remainder of its 'determinative power' (Fig. 11).
If the dorsal (embryonic) half of a blástula or a young gastrula is excised,
the ventral (extra-embryonic) half which remains rebuilds a circular
germ in which the embryo appears in its normal place, that is to say in
the place of the pre-existing dorsal zone. But when cultivated in vitro
without periblast or yolk, this zone is incapable of differentiation
(Richard etal, 1956).
Instead of simply suppressing the dorsal half, one can put the ventral
half (grafted half) of another germ in its place (as already shown in an
experiment by Luther, 1937); the embryo appears in the grafted half,
that is to say where the previous dorsal zone was. If the same type of
experiment is done with two dorsal zones, there is no competition, and
two embryos appear face to face.
Two facts must be taken into consideration: regulation of the germ
and determinism of the embryo's localization. We know, from Luther's
work (1936), that the blástula is totipotent. In the two types of experiments performed at this stage a normal embryo is formed. (See also
experiments of fissuration of the blastoderm in Salmo salvelinus; Lutz
and Lutz-Ostertag, 1952.) This totipotency has disappeared in the
gastrula, and, in these experiments, instead of a total regulation, there
is only an orderly positioning of the marginal material which is present
(regulation of the morphogenetic movements).
But in all cases, and here is the important phenomenon, the embryonic
material takes up the position in which the normal embryo would have
appeared if the germ had not been operated upon. The zone corresponding to the underlying vitelline syncytium seems to have a directing
influence and, in the case of the blástula, an organizing influence also.
At this point, and at this stage, it may be thought that part of the
crescent in its broadest portion remains and, in the absence of competition, is sufficient to make a new marginal zone appear.
In favour of the organizing influence of the 'crescent' one can quote
the result of the chronological combination (Devillers, 1952; see also
Section IV, A, 4) of a morula blastodisc with the yolk of an old gastrula
(Fig. 19): epiboly proceeds normally, but the embryo hardly organizes
itself: at the stage of epiboly 1/2 it is no more than a badly shaped