SYMMETRIZATION OF THE EGG OF VERTEBRATES 49
F. Conclusions
In birds, the plane of bilateral symmetry is directed by the rotation
of the egg in the uterus. The resulting plane varies not only with the
position of the egg in the uterus but with the position of the egg axis
in space as well. The rotations are not effective throughout the stay of
the egg in the uterus, but only up to a certain critical phase. This
critical phase coincides with the formation of the area pellucida. We can
infer that bilateral symmetry is irrevocably determined when the germ starts
off the reaction leading to the formation of this area. As yet we are unable
to ascertain the exact effect of rotation on the germ and its relation to
other reactions which take place in the egg at that time. The determining rotations may be those which take place a long time before the
critical phase. Subsequent rotations however annihilate the effects of
previous rotations. They are reversible. The rotations act as preparatory
or directive factors of symmetrization, but the latter is final only when
the formation of the area pellucida appears. The appearance of the area
pellucida is, in birds, the equivalent of the formation of the grey
crescent in amphibians. In either case, these formations indicate that
from then onwards the plane of bilateral symmetry of the germ is
irreversible.
IV. Determination of Bilateral Symmetry in Reptiles
Studies of the determination of the plane of bilateral symmetry in
reptiles are still scanty. Most of the work on the subject deals with the
relationship between the orientation of the embryo and the egg axis.
Simultaneously Pasteels (1955) in Camaesura Unguinea and Chameleo
bitaeniatus, and Clavert and Zahnd (1955) in Lacerta stirpium and
Tropidonotus natrix have shown that the rule of von Baer which
applies to these relations in birds can be applied to reptiles also, and
with the same percentage of exceptions (Fig. 10). These observations
were later confirmed by Ancel and Raynaud (1959) in Anguis fragilis.
Pasteels and ourselves came to the conclusion that the symmetrization
of the egg of reptiles took place in the uterus and that its rotation and
its movement within its membranes were the determining factors in the
orientation of the plane of bilateral symmetry. We were led to suspect
this rotation indirectly: for when the egg enters the uterus, it is comparatively round, but it becomes more and more oval while it is in the
uterus. Raynaud (1960) was able to observe directly the initial rotation
of the eggs of Anguis fragilis in the uterus and to show that the eggs
always rotate in the same direction along the two uterine horns and
that all the embryos face towards the right side of the mother. We can
F. Conclusions
In birds, the plane of bilateral symmetry is directed by the rotation
of the egg in the uterus. The resulting plane varies not only with the
position of the egg in the uterus but with the position of the egg axis
in space as well. The rotations are not effective throughout the stay of
the egg in the uterus, but only up to a certain critical phase. This
critical phase coincides with the formation of the area pellucida. We can
infer that bilateral symmetry is irrevocably determined when the germ starts
off the reaction leading to the formation of this area. As yet we are unable
to ascertain the exact effect of rotation on the germ and its relation to
other reactions which take place in the egg at that time. The determining rotations may be those which take place a long time before the
critical phase. Subsequent rotations however annihilate the effects of
previous rotations. They are reversible. The rotations act as preparatory
or directive factors of symmetrization, but the latter is final only when
the formation of the area pellucida appears. The appearance of the area
pellucida is, in birds, the equivalent of the formation of the grey
crescent in amphibians. In either case, these formations indicate that
from then onwards the plane of bilateral symmetry of the germ is
irreversible.
IV. Determination of Bilateral Symmetry in Reptiles
Studies of the determination of the plane of bilateral symmetry in
reptiles are still scanty. Most of the work on the subject deals with the
relationship between the orientation of the embryo and the egg axis.
Simultaneously Pasteels (1955) in Camaesura Unguinea and Chameleo
bitaeniatus, and Clavert and Zahnd (1955) in Lacerta stirpium and
Tropidonotus natrix have shown that the rule of von Baer which
applies to these relations in birds can be applied to reptiles also, and
with the same percentage of exceptions (Fig. 10). These observations
were later confirmed by Ancel and Raynaud (1959) in Anguis fragilis.
Pasteels and ourselves came to the conclusion that the symmetrization
of the egg of reptiles took place in the uterus and that its rotation and
its movement within its membranes were the determining factors in the
orientation of the plane of bilateral symmetry. We were led to suspect
this rotation indirectly: for when the egg enters the uterus, it is comparatively round, but it becomes more and more oval while it is in the
uterus. Raynaud (1960) was able to observe directly the initial rotation
of the eggs of Anguis fragilis in the uterus and to show that the eggs
always rotate in the same direction along the two uterine horns and
that all the embryos face towards the right side of the mother. We can