56
J. CLAVERT
explains also the alterations of the physiological properties of the egg,
particularly alterations in its permeability to dyes and in its elasticity
(Dollander, 1956). Possibly it determines a respiratory gradient which
would explain some of the results obtained by L,0vtrup (1958). This is
dealt with in greater detail by Dollander (1960) who has specially
studied and discussed these points. Finally it is the rotation of fertilization which determines the plane of symmetry in the egg, hence the
name quite rightly proposed by Ancel and Vintemberger (1948),
'rotation of symmetrization'. The external factors which direct it only
orientate the irreversible reaction which takes place at that moment.
In other amphibians, the rotation of symmetrization takes place and
fixes the plane of bilateral symmetry as in anurans, but the effective
external factor is not known, although, as we have seen, many are
possible. One can suppose, if no external factor acts on the egg, that
there may be an orientated pre-existing structure or a heterogeneity in
the distribution of the egg constituents, or a residue of the transformations that took place during activation, which would orientate the
reaction of symmetrization, but this has yet to be proved experimentally.
In birds, symmetrization is acquired as a result of the rotations which
the egg undergoes all the time that it is in the uterus, as we have shown
by several methods (Vintemberger and Clavert, 1954-1960; Clavert,
1959, 1960a-c). The resulting axis is related to the position of the egg in
the uterus (presentation) and the position of the egg axis in space.
The way the membranes rotate determines the cephalo-caudal direction
of the future embryo. The germ has a critical phase after which the
rotation of the membranes is without effect. Here again it is the position
just before this critical phase which determines the position of the plane
of symmetry, for only the last rotations have an effect and they
annihilate that of earlier rotations. The fixing of the plane of symmetry
results also from a reaction of the egg itself, which is manifested by a
change of appearance of the germ. Indeed when the area pellucida
begins its formation the directive factors lose their symmetrizing effect.
The disconnection and thinning of the central part of the blastodisc
seem to be due to an active specific reaction which we may liken to the
rotational movement of the egg cortex in amphibians. The study of this
phenomenon is not yet sufficiently advanced to state definitely what
this specific reaction is but, without doubt, it exists. It is known to
occur in the germ at the end of segmentation. Therefore in birds it is
again an external factor which intervenes to orientate the plane of
bilateral symmetry (which is fixed by a reaction of the germ).
In reptiles, very probably, the egg is symmetrized under the influence
of uterine rotations. There has been no experimental orientation of the
J. CLAVERT
explains also the alterations of the physiological properties of the egg,
particularly alterations in its permeability to dyes and in its elasticity
(Dollander, 1956). Possibly it determines a respiratory gradient which
would explain some of the results obtained by L,0vtrup (1958). This is
dealt with in greater detail by Dollander (1960) who has specially
studied and discussed these points. Finally it is the rotation of fertilization which determines the plane of symmetry in the egg, hence the
name quite rightly proposed by Ancel and Vintemberger (1948),
'rotation of symmetrization'. The external factors which direct it only
orientate the irreversible reaction which takes place at that moment.
In other amphibians, the rotation of symmetrization takes place and
fixes the plane of bilateral symmetry as in anurans, but the effective
external factor is not known, although, as we have seen, many are
possible. One can suppose, if no external factor acts on the egg, that
there may be an orientated pre-existing structure or a heterogeneity in
the distribution of the egg constituents, or a residue of the transformations that took place during activation, which would orientate the
reaction of symmetrization, but this has yet to be proved experimentally.
In birds, symmetrization is acquired as a result of the rotations which
the egg undergoes all the time that it is in the uterus, as we have shown
by several methods (Vintemberger and Clavert, 1954-1960; Clavert,
1959, 1960a-c). The resulting axis is related to the position of the egg in
the uterus (presentation) and the position of the egg axis in space.
The way the membranes rotate determines the cephalo-caudal direction
of the future embryo. The germ has a critical phase after which the
rotation of the membranes is without effect. Here again it is the position
just before this critical phase which determines the position of the plane
of symmetry, for only the last rotations have an effect and they
annihilate that of earlier rotations. The fixing of the plane of symmetry
results also from a reaction of the egg itself, which is manifested by a
change of appearance of the germ. Indeed when the area pellucida
begins its formation the directive factors lose their symmetrizing effect.
The disconnection and thinning of the central part of the blastodisc
seem to be due to an active specific reaction which we may liken to the
rotational movement of the egg cortex in amphibians. The study of this
phenomenon is not yet sufficiently advanced to state definitely what
this specific reaction is but, without doubt, it exists. It is known to
occur in the germ at the end of segmentation. Therefore in birds it is
again an external factor which intervenes to orientate the plane of
bilateral symmetry (which is fixed by a reaction of the germ).
In reptiles, very probably, the egg is symmetrized under the influence
of uterine rotations. There has been no experimental orientation of the