SYMMETRIZATION OF THE EGG OF VERTEBRATES 47
To conclude, the presentation of the egg in the uterus orientates the
cephalo-caudal axis in general, and the position in space of the egg axis
fixes the plane of bilateral symmetry. The empirical rule of von Baer can
be explained by the fact that presentation with the sharp end first and
the horizontal position of the egg axis in the uterus are the most
frequent in birds, particularly in the hen.
E. The Critical Phase during Symmetrization of the Uterine Egg
We have seen in the preceding chapter that when an egg remains with
its sharp end first throughout its stay in the uterus, the head of the
embryo is always directed away from the observer when its position is
determined according to the rule of von Baer. On the contrary if the egg
changes its presentation from sharp end first to blunt end first, and
stays thus until it is laid, the orientation of the embryo is reversed.
These observations indicate that the orientation of the embryo is not
final in the uterine egg. In the duck (Clavert and Vintemberger, 1954a, b)
by taking repeated X-ray photographs, we observed spontaneous
changes of presentation, which were late and isolated (during the last
6 hours in the uterus) and without effect on symmetrization. These
results indicated the existence of a critical phase during the process of
symmetrization (Vintemberger and Clavert, 1959; Clavert, 1961). We
were able to establish that if we turned hens' eggs through 180° in the
uterus at least 8 hours before they were laid, we obtained embryos whose
orientation was opposite to that defined by the rule of von Baer. If the eggs
were turned less than 6 hours before they were laid, there was no effect.
During its stay in the uterus, the egg therefore loses its ability to
symmetrize. The period from 8 to 6 hours before the egg is laid corresponds to the period during which the germ becomes transformed
(Clavert and Vintemberger, 1960; Clavert, 1961); it detaches itself from
the vitellus, becomes thinner and spreads over the yolk surface. The
area pellucida then appears, off-centre (Fig. 9). Further development
shows that the plane so established is the final plane of bilateral
symmetry. When this area appears, the cephalo-caudal axis is fixed.
Similar results were obtained for the pigeon, although they differed
chronologically (Vintemberger and Clavert, 1955).
We can therefore infer that in birds bilateral symmetry is achieved
when the area pellucida is formed; and it is the egg position in the
uterus before this critical period that is determining. In vitro experiments gave spectacular proof of these conclusions (Vintemberger and
Clavert, 1960). If uterine eggs are removed 10 to 12 hours before the
presumed laying time and are placed within a cylinder rotating at about
the same speed as the egg in the uterus (10-12 revolutions per hour),
the cephalo-caudal axis of the embryo can be directed at will. One only
To conclude, the presentation of the egg in the uterus orientates the
cephalo-caudal axis in general, and the position in space of the egg axis
fixes the plane of bilateral symmetry. The empirical rule of von Baer can
be explained by the fact that presentation with the sharp end first and
the horizontal position of the egg axis in the uterus are the most
frequent in birds, particularly in the hen.
E. The Critical Phase during Symmetrization of the Uterine Egg
We have seen in the preceding chapter that when an egg remains with
its sharp end first throughout its stay in the uterus, the head of the
embryo is always directed away from the observer when its position is
determined according to the rule of von Baer. On the contrary if the egg
changes its presentation from sharp end first to blunt end first, and
stays thus until it is laid, the orientation of the embryo is reversed.
These observations indicate that the orientation of the embryo is not
final in the uterine egg. In the duck (Clavert and Vintemberger, 1954a, b)
by taking repeated X-ray photographs, we observed spontaneous
changes of presentation, which were late and isolated (during the last
6 hours in the uterus) and without effect on symmetrization. These
results indicated the existence of a critical phase during the process of
symmetrization (Vintemberger and Clavert, 1959; Clavert, 1961). We
were able to establish that if we turned hens' eggs through 180° in the
uterus at least 8 hours before they were laid, we obtained embryos whose
orientation was opposite to that defined by the rule of von Baer. If the eggs
were turned less than 6 hours before they were laid, there was no effect.
During its stay in the uterus, the egg therefore loses its ability to
symmetrize. The period from 8 to 6 hours before the egg is laid corresponds to the period during which the germ becomes transformed
(Clavert and Vintemberger, 1960; Clavert, 1961); it detaches itself from
the vitellus, becomes thinner and spreads over the yolk surface. The
area pellucida then appears, off-centre (Fig. 9). Further development
shows that the plane so established is the final plane of bilateral
symmetry. When this area appears, the cephalo-caudal axis is fixed.
Similar results were obtained for the pigeon, although they differed
chronologically (Vintemberger and Clavert, 1955).
We can therefore infer that in birds bilateral symmetry is achieved
when the area pellucida is formed; and it is the egg position in the
uterus before this critical period that is determining. In vitro experiments gave spectacular proof of these conclusions (Vintemberger and
Clavert, 1960). If uterine eggs are removed 10 to 12 hours before the
presumed laying time and are placed within a cylinder rotating at about
the same speed as the egg in the uterus (10-12 revolutions per hour),
the cephalo-caudal axis of the embryo can be directed at will. One only