SYMMETRIZATION OF THE EGG OF VERTEBRATES
J . C L A V E R T
Professor of the Faculty of Medicine, Strasbourg, France
I. Introduction
27
II. Determination of Bilateral »Symmetry in Amphibians
28
A. Ranafusca
28
B. Other Amphibians
37
C. Conclusions
38
III. Determination of Bilateral Symmetry in Birds
39
A. Structure of the Egg
39
B. Formation of the Egg
41
C. Relation between the Position of the Embryo and Direction of Coiling of
Chalazae
42
D. Relation between Egg Position in the Uterus and Embryo Orientation
43
E. The Critical Phase during Symmetrization of the Uterine Egg
47
F. Conclusions
49
IV. Determination of Bilateral Symmetry in Reptiles
49
V. Determination of Bilateral Symmetry in Fish
51
A. Chondrostei (Acipenser sturio, A. ruthenus, A. huso)
51
B. Selachii (Scylliorhinus canicula)
53
C. Teleostei
54
VI. Discussion and Interpretation of Results
55
VII. Summary
59
References
59
I. Introduction
The egg of vertebrates is a voluminous spherical or oval cell, with an
axis of polarity represented by the diameter joining the animal pole to
the vegetal pole. Around this axis, as we shall see further on, the
constituent materials of the egg are distributed evenly. During development, a particular meridian defines itself. The plane that passes through
this meridian and contains the axis of polarity becomes the plane of
bilateral symmetry. It divides the egg into two parts which will define
the future right and left sides of the embryo; the cephalo-caudal axis
lies along this meridian. The study of symmetrization consists essentially in the study of the determination of this preferential meridian
during development. Is it pre-formed, or if not, what are its determining
factors? How does it become defined? Such are the questions which we
shall try to answer.
The problem of the symmetrization of the vertebrate egg has not been
studied extensively and the work devoted to it is unevenly distributed
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