IX. DEVELOPMENT OF THE TELEOSTEAN EGG
415
vitelline surface,
c
non-wettable' by the blastoderm and the inner side of
the blastoderm.
In Salmo, the suppresssion of the blastoderm at the same stages is
very generally followed by a syncytial retraction. It is probably because
of the greater thickness of the peripheral syncytial ridge that the force
of retraction of the ring caused by the medium (see Section III, D) overcomes the force of spreading. If this case is compared to Fundulus there
is only a quantitative difference between the forces present, not a total
difference, since with some batches of Salmo it is possible to obtain the
same results as in Fundulus.
In the spreading syncytium, the peripheral ridge thus keeps its
faculty of responding by a contraction to contact with water. In Salmo,
but not in Fundulus (Trinkaus, 1949b), this contraction is strong
enough to oppose epiboly.
Such a phenomenon can only reveal itself if the impermeable covering
of the egg is artificially altered.
Thus the opening of the roof of the blastocoel causes a retraction of
the blastoderm which only ends when the wound is closed; then
spreading starts again. The same retraction can be observed after
scraping the enveloping layer.
Here we have an exogastrulation, limited to the yolk plug alone
(non-segmented equivalent of the sub-blastoporal material of Amphibia) ; a total exogastrulation with the marginal material moving out
has never been observed. This difference in behaviour between the eggs
of Amphibia and those of Fishes must be due to the fact that in the
latter the marginal material invaginates in a special way (see Section
IV, B) and cannot reverse the process afterwards.
The disorders of epiboly caused by hypertonic solutions (Holtfreter's
solution concentrated six times or NaCl at 21 per cent) or by detergents
(Duponol G at 0-5 mg per cent) (Devillers, 1948b, 1950a on Salmo;
Thomopoulos, 1955, on Perca) must also be the result of the contraction
of the syncytial ring, due to a more complex mechanism.
Anionic detergents at a biological pH alter the structure of the
covering (Runnström et al., 1945); NaCl weakens interionic attractions
and increases permeability. On one hand, the covering passes from a
solid state to a more fluid one, weakening its spreading capacity; on the
other hand, water can enter and start the syncytial contraction.
When applied before the crossing of the equator, the hypertonic
treatment causes an exogastrulation which can be explained by the
syncytial ridge's contraction alone. When applied after the crossing of
the equator, this treatment only causes epiboly to stop. If the action on
the syncytial ring alone were involved, the contraction, directed this
time towards the lower pole, should speed up epiboly. It must be
415
vitelline surface,
c
non-wettable' by the blastoderm and the inner side of
the blastoderm.
In Salmo, the suppresssion of the blastoderm at the same stages is
very generally followed by a syncytial retraction. It is probably because
of the greater thickness of the peripheral syncytial ridge that the force
of retraction of the ring caused by the medium (see Section III, D) overcomes the force of spreading. If this case is compared to Fundulus there
is only a quantitative difference between the forces present, not a total
difference, since with some batches of Salmo it is possible to obtain the
same results as in Fundulus.
In the spreading syncytium, the peripheral ridge thus keeps its
faculty of responding by a contraction to contact with water. In Salmo,
but not in Fundulus (Trinkaus, 1949b), this contraction is strong
enough to oppose epiboly.
Such a phenomenon can only reveal itself if the impermeable covering
of the egg is artificially altered.
Thus the opening of the roof of the blastocoel causes a retraction of
the blastoderm which only ends when the wound is closed; then
spreading starts again. The same retraction can be observed after
scraping the enveloping layer.
Here we have an exogastrulation, limited to the yolk plug alone
(non-segmented equivalent of the sub-blastoporal material of Amphibia) ; a total exogastrulation with the marginal material moving out
has never been observed. This difference in behaviour between the eggs
of Amphibia and those of Fishes must be due to the fact that in the
latter the marginal material invaginates in a special way (see Section
IV, B) and cannot reverse the process afterwards.
The disorders of epiboly caused by hypertonic solutions (Holtfreter's
solution concentrated six times or NaCl at 21 per cent) or by detergents
(Duponol G at 0-5 mg per cent) (Devillers, 1948b, 1950a on Salmo;
Thomopoulos, 1955, on Perca) must also be the result of the contraction
of the syncytial ring, due to a more complex mechanism.
Anionic detergents at a biological pH alter the structure of the
covering (Runnström et al., 1945); NaCl weakens interionic attractions
and increases permeability. On one hand, the covering passes from a
solid state to a more fluid one, weakening its spreading capacity; on the
other hand, water can enter and start the syncytial contraction.
When applied before the crossing of the equator, the hypertonic
treatment causes an exogastrulation which can be explained by the
syncytial ridge's contraction alone. When applied after the crossing of
the equator, this treatment only causes epiboly to stop. If the action on
the syncytial ring alone were involved, the contraction, directed this
time towards the lower pole, should speed up epiboly. It must be