32
J. CLAVERT
bilateral symmetry. The grey crescent resulting from the rotation of
fertilization is a region, originally black, from which the pigment has
been partially swept. The deformed sperm track on the ventral side
confirms the existence of a rotatory movement of the cortex, for
peripherally it becomes bent downwards towards the vegetal pole.
This rotation at a temperature of 18° starts at 70 minutes after fertilization and lasts for about an hour. The symmetrized egg also presents
important ultra-structural modifications: the peripheral micro villi
disappear and a dense layer, which did not exist beforehand, forms at the
periphery of the egg, immediately under the vitelline membrane. The
modifications of the cortex cause the dorsal part of the vitelline
cupola to come nearer to the vitelline membrane. A trail of yolk
granules is detached from this side of the vitelline cupola, and is carried
towards the animal pole by the movement of the cortical layer. We shall
see that this movement of the cortical layer is a fundamental reaction in
determining the plane of bilateral symmetry.
3. Role of the Spermatozoon
Newport (1854) was the first to point out a possible relation between
the point of penetration of the spermatozoon and the position of the
plane of bilateral symmetry of the embryo. He had noticed, when
placing spermatic fluid on a freshly removed frog's egg that had been
fixed to a glass slide, that the meridian of fertilization frequently
coincided with the plane of bilateral symmetry of the embryo.
Roux (1887), repeating similar experiments on eggs of Rana fusca
was able to prove that these eggs could be fertilized at will, in any
meridian whatsoever, and that the first cleavage plane, together with
the plane of symmetry of the embryo, passed through the point of
entrance of the spermatozoon. The coincidence between the first
cleavage plane and the plane of bilateral symmetry was much discussed,
but many research workers (Schultze, 1899; Hertwig, 1894; Morgan and
Boring, 1903; Morgan and Tsuda, 1894; Brächet, 1903; Ancel and
Vintemberger, 1948), though with various reservations, agreed that the
coincidence existed and therefore considered the work of Roux to be
valid. In particular, the demonstrative experiments of Brächet (1903)
and of Ancel and Vintemberger (1948) substantially confirmed the
conclusions of Newport and Roux. Brächet (1903) had in fact verified
that in normal conditions the sperm track was always to be found in the
plane of bilateral symmetry and on the ventral side. Ancel and Vintemberger confirmed these conclusions by observations on eggs obtained
after spontaneous ovulation, in which the authors systematically noted
the position of the sperm track in relation to the centre of the grey
crescent, and on eggs after localized fertilization performed by their
J. CLAVERT
bilateral symmetry. The grey crescent resulting from the rotation of
fertilization is a region, originally black, from which the pigment has
been partially swept. The deformed sperm track on the ventral side
confirms the existence of a rotatory movement of the cortex, for
peripherally it becomes bent downwards towards the vegetal pole.
This rotation at a temperature of 18° starts at 70 minutes after fertilization and lasts for about an hour. The symmetrized egg also presents
important ultra-structural modifications: the peripheral micro villi
disappear and a dense layer, which did not exist beforehand, forms at the
periphery of the egg, immediately under the vitelline membrane. The
modifications of the cortex cause the dorsal part of the vitelline
cupola to come nearer to the vitelline membrane. A trail of yolk
granules is detached from this side of the vitelline cupola, and is carried
towards the animal pole by the movement of the cortical layer. We shall
see that this movement of the cortical layer is a fundamental reaction in
determining the plane of bilateral symmetry.
3. Role of the Spermatozoon
Newport (1854) was the first to point out a possible relation between
the point of penetration of the spermatozoon and the position of the
plane of bilateral symmetry of the embryo. He had noticed, when
placing spermatic fluid on a freshly removed frog's egg that had been
fixed to a glass slide, that the meridian of fertilization frequently
coincided with the plane of bilateral symmetry of the embryo.
Roux (1887), repeating similar experiments on eggs of Rana fusca
was able to prove that these eggs could be fertilized at will, in any
meridian whatsoever, and that the first cleavage plane, together with
the plane of symmetry of the embryo, passed through the point of
entrance of the spermatozoon. The coincidence between the first
cleavage plane and the plane of bilateral symmetry was much discussed,
but many research workers (Schultze, 1899; Hertwig, 1894; Morgan and
Boring, 1903; Morgan and Tsuda, 1894; Brächet, 1903; Ancel and
Vintemberger, 1948), though with various reservations, agreed that the
coincidence existed and therefore considered the work of Roux to be
valid. In particular, the demonstrative experiments of Brächet (1903)
and of Ancel and Vintemberger (1948) substantially confirmed the
conclusions of Newport and Roux. Brächet (1903) had in fact verified
that in normal conditions the sperm track was always to be found in the
plane of bilateral symmetry and on the ventral side. Ancel and Vintemberger confirmed these conclusions by observations on eggs obtained
after spontaneous ovulation, in which the authors systematically noted
the position of the sperm track in relation to the centre of the grey
crescent, and on eggs after localized fertilization performed by their