28
J. CLAVERT
amongst the vertebrates. Amphibians and birds have been the object of
the most intensive work. They will therefore play a prominent part in
this report. The available information on the other classes is only
fragmentary and hardly sufficient to permit a generalized picture of
egg symmetrization in the vertebrates as a whole. In the course of this
chapter we will review, therefore, the phenomena of symmetrization in
amphibia, birds, reptiles, and fish before coming to the discussion and
conclusion to which this survey leads.
II. Determination of Bilateral Symmetry in Amphibians
Newport and Roux (1854, 1887) were the first to demonstrate that it
was possible to influence experimentally the determination of the plane
of bilateral symmetry in amphibians. Their experiments and observations showed that the plane of bilateral symmetry is not definitely preformed. Further experiments confirmed and widened these early
observations, which were made on the egg of Rana fusca, which is
admirably suited to experimentation. The results of Ancel and Vintemberger (1948) were to throw a new light upon the problem of the
symmetrization of this egg; the greater part of this discussion will
therefore deal with symmetrization of the egg of Rana fusca. Later we
shall review some of the scattered facts relating to other amphibians and
so obtain a general notion of the determination of symmetrization in
the whole class.
A. Rana fusca
1. Structure of the Unfertilized Egg
To understand the way in which the plane of bilateral symmetry is
determined in Rana fusca, it is necessary to describe the structure of the
unfertilized egg, the modifications which it undergoes during the early
stages of development, and the factors which act upon the determination
of the plane of symmetry. First, let us examine the structure of the
unfertilized egg.
The unfertilized egg of Rana fusca (Fig. 1), according to Ancel and
Vintemberger (1948), is a sphere of 2 mm in diameter, which outwardly
presents two regions, one dark brown, the other whitish or grey. The
first occupies the animal half of the egg and extends into the subequatorial region. The other zone, which does not possess any superficial
pigment, makes up the vitelline field. The difference of colour is due to
the presence of a layer of black pigment under the vitelline membrane
of the upper zone. At the superior pole there is a small area where the
pigmented layer is thinner: this is the superior polar spot. Here are
found the first polar body and a small centrally placed depression which
J. CLAVERT
amongst the vertebrates. Amphibians and birds have been the object of
the most intensive work. They will therefore play a prominent part in
this report. The available information on the other classes is only
fragmentary and hardly sufficient to permit a generalized picture of
egg symmetrization in the vertebrates as a whole. In the course of this
chapter we will review, therefore, the phenomena of symmetrization in
amphibia, birds, reptiles, and fish before coming to the discussion and
conclusion to which this survey leads.
II. Determination of Bilateral Symmetry in Amphibians
Newport and Roux (1854, 1887) were the first to demonstrate that it
was possible to influence experimentally the determination of the plane
of bilateral symmetry in amphibians. Their experiments and observations showed that the plane of bilateral symmetry is not definitely preformed. Further experiments confirmed and widened these early
observations, which were made on the egg of Rana fusca, which is
admirably suited to experimentation. The results of Ancel and Vintemberger (1948) were to throw a new light upon the problem of the
symmetrization of this egg; the greater part of this discussion will
therefore deal with symmetrization of the egg of Rana fusca. Later we
shall review some of the scattered facts relating to other amphibians and
so obtain a general notion of the determination of symmetrization in
the whole class.
A. Rana fusca
1. Structure of the Unfertilized Egg
To understand the way in which the plane of bilateral symmetry is
determined in Rana fusca, it is necessary to describe the structure of the
unfertilized egg, the modifications which it undergoes during the early
stages of development, and the factors which act upon the determination
of the plane of symmetry. First, let us examine the structure of the
unfertilized egg.
The unfertilized egg of Rana fusca (Fig. 1), according to Ancel and
Vintemberger (1948), is a sphere of 2 mm in diameter, which outwardly
presents two regions, one dark brown, the other whitish or grey. The
first occupies the animal half of the egg and extends into the subequatorial region. The other zone, which does not possess any superficial
pigment, makes up the vitelline field. The difference of colour is due to
the presence of a layer of black pigment under the vitelline membrane
of the upper zone. At the superior pole there is a small area where the
pigmented layer is thinner: this is the superior polar spot. Here are
found the first polar body and a small centrally placed depression which