SYMMETRIZATION OF THE EGG OF VERTEBRATES 39
rotation of fertilization, when the plane of bilateral symmetry of the
egg is fixed irreversibly. The rotation of fertilization is a shift of the
cortex over the deeper layers. It corresponds most probably to one or
several irreversible physico-chemical modifications which take place in
the egg; the study of ultra-structure seems to confirm this without
giving any precise information as to the nature of the modifications.
Therefore the rotation of fertilization which determines the symmetrization of the egg is a specific reaction.
All these factors act upon symmetrization from the outside to
orientate the rotation of fertilization, but their action is always reversible and it is the last factor to act before the rotation of fertilization
that is determining. Ancel and Vintemberger proposed to name these
factors 'preparatory factors' of symmetrization. Their action may be
exerted long before the beginning of the rotation of fertilization, but if
no other action intervenes, the preparatory factor is determining.
Their precise mode of action is not known; one can only point out that
they result in pigment asymmetry. One can assume that the organization of the egg or the initial reactions which take place immediately
after fertilization may have the same action as the preparatory factors
(if no other factor intervenes).
Finally, in amphibians, the rotation of fertilization fixes the plane of
bilateral symmetry of the embryo and, quite correctly, Ancel and Vintemberger (1948) proposed to call it the rotation of symmetrization.
III. Determination of Bilateral Symmetry in Birds
The only early work concerning symmetrization in birds is that of
Bartelmez (1912-1918) on the chicken. From his purely morphological
study he infers that the axis of the embryo is preformed in the ovary,
thus explaining the empirically elaborated rule of von Baer. With
Vintemberger (1954-1960) we have brought forward many concordant
experimental demonstrations of the epigenetic nature of the determination of bilateral symmetry in birds. Here again, as in amphibians, it is
necessary to know in detail the structure and the mode of formation of
the egg in order to understand how symmetrization takes place.
A. Structure of the Egg
The eggs of birds are all of the same type: in the centre is the yolk or
egg proper surrounded by its membrane, the vitelline membrane, and
the albumen.
The structure has been investigated in more detail in the work of
Asmundson (1931), Bartelmez (1918), Conrad and Scott (1938), Conrad
and Philipps (1938), Marza (1938), Pearl and Curtis (1912), Romanoff
rotation of fertilization, when the plane of bilateral symmetry of the
egg is fixed irreversibly. The rotation of fertilization is a shift of the
cortex over the deeper layers. It corresponds most probably to one or
several irreversible physico-chemical modifications which take place in
the egg; the study of ultra-structure seems to confirm this without
giving any precise information as to the nature of the modifications.
Therefore the rotation of fertilization which determines the symmetrization of the egg is a specific reaction.
All these factors act upon symmetrization from the outside to
orientate the rotation of fertilization, but their action is always reversible and it is the last factor to act before the rotation of fertilization
that is determining. Ancel and Vintemberger proposed to name these
factors 'preparatory factors' of symmetrization. Their action may be
exerted long before the beginning of the rotation of fertilization, but if
no other action intervenes, the preparatory factor is determining.
Their precise mode of action is not known; one can only point out that
they result in pigment asymmetry. One can assume that the organization of the egg or the initial reactions which take place immediately
after fertilization may have the same action as the preparatory factors
(if no other factor intervenes).
Finally, in amphibians, the rotation of fertilization fixes the plane of
bilateral symmetry of the embryo and, quite correctly, Ancel and Vintemberger (1948) proposed to call it the rotation of symmetrization.
III. Determination of Bilateral Symmetry in Birds
The only early work concerning symmetrization in birds is that of
Bartelmez (1912-1918) on the chicken. From his purely morphological
study he infers that the axis of the embryo is preformed in the ovary,
thus explaining the empirically elaborated rule of von Baer. With
Vintemberger (1954-1960) we have brought forward many concordant
experimental demonstrations of the epigenetic nature of the determination of bilateral symmetry in birds. Here again, as in amphibians, it is
necessary to know in detail the structure and the mode of formation of
the egg in order to understand how symmetrization takes place.
A. Structure of the Egg
The eggs of birds are all of the same type: in the centre is the yolk or
egg proper surrounded by its membrane, the vitelline membrane, and
the albumen.
The structure has been investigated in more detail in the work of
Asmundson (1931), Bartelmez (1918), Conrad and Scott (1938), Conrad
and Philipps (1938), Marza (1938), Pearl and Curtis (1912), Romanoff