SYMMETRIZATION OF THE EGG OF VERTEBRATES 45
All the duck, hen, and pigeon eggs which we studied in this way
showed that if the egg presents itself with its sharp end facing the cloaca
during its intra-uterine stay ('sharp end first') the head of the embryo is
always on the side away from the observer (in zone S). If on the other
hand, the blunt end faces the cloaca ('blunt end first') the head of
the embryo is always directed towards the observer (in zone I). One
must add that the cephalo-caudal axis in embryos may vary by as much
as 90° to the right or to the left of the standard position of von Baer
and of the reverse position.
In the hen and the pigeon, we manipulated the egg in the uterus,
turning it through 180°; if this was done at the beginning of the
uterine phase, we obtained an inversion of the orientation of the
embryo. It can be concluded that egg presentation in the uterus orientates
the cephalo-caudal axis of the embryo. However it does not explain the
exact position of the cephalo-caudal axis: another factor is involved in
this.
2. Influence of the Position of the Egg Axis
In the preceding section we described the influence of the two modes
of egg presentation in the uterus on the orientation of the embryo.
To each presentation there is, however, an infinite number of possible
planes of symmetry.
Clavert and Vintemberger (1958) and Clavert (1960a-c), considered
what other factors might intervene to determine one particular plane
from all others. Statistical analysis of the orientations of embryos in the
eggs of the hen, duck, goose, pigeon, and turkey revealed a dominant
orientation of the axis of the embryo for a given species or individual
parent. In the hen, the duck and the goose the dominant orientation is true to the rule of von Baer. A majority of embryos therefore
have their cephalo-caudal axis in a plane perpendicular to the egg
axis. Displacements to the right or to the left of this position are nearly
equal in frequency in the duck and the hen, but are more frequent to
the left (10 : 1) in the goose. The axes of pigeon embryos have, on the
contrary, a dominant orientation offset 30° to the right. In the turkey
there are interesting individual variations. In some individuals the
cephalo-caudal axis of the embryo always followed exactly the rule of
von Baer; in other individuals, invariably the axes were offset noticeably to the right (45° on the average).
We were able to establish statistically that there was a relationship
between the position of the egg axis in the uterus and the final position
of the cephalo-caudal axis of the embryo. By making many profile
X-ray photographs of birds one can get an accurate three-dimensional
picture of the position of the egg axis in the uterus. We have found that
All the duck, hen, and pigeon eggs which we studied in this way
showed that if the egg presents itself with its sharp end facing the cloaca
during its intra-uterine stay ('sharp end first') the head of the embryo is
always on the side away from the observer (in zone S). If on the other
hand, the blunt end faces the cloaca ('blunt end first') the head of
the embryo is always directed towards the observer (in zone I). One
must add that the cephalo-caudal axis in embryos may vary by as much
as 90° to the right or to the left of the standard position of von Baer
and of the reverse position.
In the hen and the pigeon, we manipulated the egg in the uterus,
turning it through 180°; if this was done at the beginning of the
uterine phase, we obtained an inversion of the orientation of the
embryo. It can be concluded that egg presentation in the uterus orientates
the cephalo-caudal axis of the embryo. However it does not explain the
exact position of the cephalo-caudal axis: another factor is involved in
this.
2. Influence of the Position of the Egg Axis
In the preceding section we described the influence of the two modes
of egg presentation in the uterus on the orientation of the embryo.
To each presentation there is, however, an infinite number of possible
planes of symmetry.
Clavert and Vintemberger (1958) and Clavert (1960a-c), considered
what other factors might intervene to determine one particular plane
from all others. Statistical analysis of the orientations of embryos in the
eggs of the hen, duck, goose, pigeon, and turkey revealed a dominant
orientation of the axis of the embryo for a given species or individual
parent. In the hen, the duck and the goose the dominant orientation is true to the rule of von Baer. A majority of embryos therefore
have their cephalo-caudal axis in a plane perpendicular to the egg
axis. Displacements to the right or to the left of this position are nearly
equal in frequency in the duck and the hen, but are more frequent to
the left (10 : 1) in the goose. The axes of pigeon embryos have, on the
contrary, a dominant orientation offset 30° to the right. In the turkey
there are interesting individual variations. In some individuals the
cephalo-caudal axis of the embryo always followed exactly the rule of
von Baer; in other individuals, invariably the axes were offset noticeably to the right (45° on the average).
We were able to establish statistically that there was a relationship
between the position of the egg axis in the uterus and the final position
of the cephalo-caudal axis of the embryo. By making many profile
X-ray photographs of birds one can get an accurate three-dimensional
picture of the position of the egg axis in the uterus. We have found that