SYMMETRIZATION OF THE EGG OF VERTEBRATES 43
eggs, 87 turkey eggs, and 200 hen eggs, that there was a close relation
between the direction of the coil in clearly identified chalazae and the
orientation of the embryo (Fig. 6). In nearly every case, the anticlockwise chalaza was at the right of the embryo and the clockwise
chalaza at the left. The only exception, in these four species, to this
fundamental rule occurs when the axis of the embryo is situated exactly
along the axis of the chalazae. The duck gave less consistent results;
of 300 duck eggs examined we noted, first, a great number of embryos
whose orientation was contrary to the rule of von Baer (confirming the
observations of Fere, 1896, and Micek, 1955), and secondly a large
number of chalazae whose coil was unrecognizable. In eggs having
recognizable coiling, the relations were constant, except in 9 cases out
of 203. In these 9 cases, the arrangement was opposite to the normal.
Furthermore the movement of the membranes which determines the
coiling of the chalazae is caudo-cephalic in most cases.
To conclude, the relationship is clear though not absolute between the
direction of coiling of the chalazae and the orientation of the bird
embryo. As a rule, the anticlockwise chalaza is at the right of the
embryo and the clockwise chalaza at its left. The movement of the
membranes which determines this coil is normally cephalo-caudal.
Certain observations on the duck suggest however that the behaviour
of the egg in the genital tract is different from that of other birds.
D. Relation between Egg Position in the Uterus and Embryo Orientation
1. Influence of Egg Presentation in the Uterus
The results of the experiments of Olsen and Byerly (1935) show that
the hen egg can present itself in two possible ways in the uterus, either
with the broad end or with the sharp end facing the cloaca (Fig. 7).
Since the egg always rotates in the same direction, the direction of the
chalazae spiral therefore depends on the type of egg presentation.
Vintemberger and Clavert (1954a-c) and Clavert (1960a-c) came to the
conclusion that these two types of egg presentation could play a
determining part in the orientation of the future embryo, owing to the
relationship between the direction of coiling of the chalazae and
the orientation of the embryo just described. We studied in detail,
therefore, egg presentation in the uterus, by taking X-ray photographs dorsally at regular intervals. On examination of the incubated
eggs (following the rule of von Baer, with the sharp end towards the
right of the observer) the vertical plane passing through the cephalo -
caudal axis of the embryo determines two regions in the upper hemisphere of the yolk, one towards the observer, which we called zone I, and
the other away from the observer which we called zone S.
eggs, 87 turkey eggs, and 200 hen eggs, that there was a close relation
between the direction of the coil in clearly identified chalazae and the
orientation of the embryo (Fig. 6). In nearly every case, the anticlockwise chalaza was at the right of the embryo and the clockwise
chalaza at the left. The only exception, in these four species, to this
fundamental rule occurs when the axis of the embryo is situated exactly
along the axis of the chalazae. The duck gave less consistent results;
of 300 duck eggs examined we noted, first, a great number of embryos
whose orientation was contrary to the rule of von Baer (confirming the
observations of Fere, 1896, and Micek, 1955), and secondly a large
number of chalazae whose coil was unrecognizable. In eggs having
recognizable coiling, the relations were constant, except in 9 cases out
of 203. In these 9 cases, the arrangement was opposite to the normal.
Furthermore the movement of the membranes which determines the
coiling of the chalazae is caudo-cephalic in most cases.
To conclude, the relationship is clear though not absolute between the
direction of coiling of the chalazae and the orientation of the bird
embryo. As a rule, the anticlockwise chalaza is at the right of the
embryo and the clockwise chalaza at its left. The movement of the
membranes which determines this coil is normally cephalo-caudal.
Certain observations on the duck suggest however that the behaviour
of the egg in the genital tract is different from that of other birds.
D. Relation between Egg Position in the Uterus and Embryo Orientation
1. Influence of Egg Presentation in the Uterus
The results of the experiments of Olsen and Byerly (1935) show that
the hen egg can present itself in two possible ways in the uterus, either
with the broad end or with the sharp end facing the cloaca (Fig. 7).
Since the egg always rotates in the same direction, the direction of the
chalazae spiral therefore depends on the type of egg presentation.
Vintemberger and Clavert (1954a-c) and Clavert (1960a-c) came to the
conclusion that these two types of egg presentation could play a
determining part in the orientation of the future embryo, owing to the
relationship between the direction of coiling of the chalazae and
the orientation of the embryo just described. We studied in detail,
therefore, egg presentation in the uterus, by taking X-ray photographs dorsally at regular intervals. On examination of the incubated
eggs (following the rule of von Baer, with the sharp end towards the
right of the observer) the vertical plane passing through the cephalo -
caudal axis of the embryo determines two regions in the upper hemisphere of the yolk, one towards the observer, which we called zone I, and
the other away from the observer which we called zone S.