406
CH.
DEVILLERS
of water the peripheral ring contracts rapidly at first and then slowly,
and complete closing up takes about 24 hours (in a physiological solution). The perivitelline cortex covers the entire vitelline sphere, which
can be replaced in ordinary water and survive there for a few days.
Here we find again the closing-up process of a wound in which the
peripheral syncytial ring plays the main part (see the plasmagel ridge
of Lewis, Section III, C, 2, d).
The vigour of the syncytial contraction varies somewhat from one
egg-laying to another, and often within the same batch of eggs, making
it difficult to obtain significant results on the action of different media
upon this process.
The contraction is sometimes strong enough to bring about a closingup in distilled water (although the syncytial surface is not impermeable).
Monovalent ions (isotonic Holtfreter's solution lacking CaCl 2 ) lower the
strength of the contraction without suppressing it; Ca has an antagonistic action. In the intact egg the syncytial tension does not reveal itself
because of the protection given by the impermeable enveloping layer;
it is only induced by water acting on the cytoplasm. This explains why
a large wound on the surface of the blastodisc causes its retraction until
cicatrization is complete; made on a germ of Salmo undergoing epiboly,
the spreading stops, regresses, then starts again after the healing.
During development, the syncytium plays a triple part:
(1) in the mobilization of yolk reserves by enzyme secretion (van der
Ghinst, 1935);
(2) as a zone of passage of the periblastic material on its way towards
the blastodisc (determination of the marginal zone). Up to the blástula
stage, the syncytium buds off blastomeres, probably filled with this
determining material, that join on to those that form the blastoderm.
After this stage, budding ceases, but the study of symmetrization (see
Section III, B, 2) shows that the syncytium continues to act upon the
blastoderm;
(3) when gastrulation starts, the syncytium shows a property of
autonomous spreading which is one of the chief factors in epiboly of
Fundulus according to Trinkaus (1949a, 1951, 1952a, b). Epiboly might
be compared to a 'wetting' where the syncytium plays an intermediate
role between the deep surface of the blastoderm and the 'non-wettable'
vitelline surface. During this expansion, the syncytial ring does not lose
its contractility.
IV. Mechanics of Gastrulation
When premorphogenesis comes to an end, the egg has finished acquiring its structure: the separation between cytoplasm and yolk is
CH.
DEVILLERS
of water the peripheral ring contracts rapidly at first and then slowly,
and complete closing up takes about 24 hours (in a physiological solution). The perivitelline cortex covers the entire vitelline sphere, which
can be replaced in ordinary water and survive there for a few days.
Here we find again the closing-up process of a wound in which the
peripheral syncytial ring plays the main part (see the plasmagel ridge
of Lewis, Section III, C, 2, d).
The vigour of the syncytial contraction varies somewhat from one
egg-laying to another, and often within the same batch of eggs, making
it difficult to obtain significant results on the action of different media
upon this process.
The contraction is sometimes strong enough to bring about a closingup in distilled water (although the syncytial surface is not impermeable).
Monovalent ions (isotonic Holtfreter's solution lacking CaCl 2 ) lower the
strength of the contraction without suppressing it; Ca has an antagonistic action. In the intact egg the syncytial tension does not reveal itself
because of the protection given by the impermeable enveloping layer;
it is only induced by water acting on the cytoplasm. This explains why
a large wound on the surface of the blastodisc causes its retraction until
cicatrization is complete; made on a germ of Salmo undergoing epiboly,
the spreading stops, regresses, then starts again after the healing.
During development, the syncytium plays a triple part:
(1) in the mobilization of yolk reserves by enzyme secretion (van der
Ghinst, 1935);
(2) as a zone of passage of the periblastic material on its way towards
the blastodisc (determination of the marginal zone). Up to the blástula
stage, the syncytium buds off blastomeres, probably filled with this
determining material, that join on to those that form the blastoderm.
After this stage, budding ceases, but the study of symmetrization (see
Section III, B, 2) shows that the syncytium continues to act upon the
blastoderm;
(3) when gastrulation starts, the syncytium shows a property of
autonomous spreading which is one of the chief factors in epiboly of
Fundulus according to Trinkaus (1949a, 1951, 1952a, b). Epiboly might
be compared to a 'wetting' where the syncytium plays an intermediate
role between the deep surface of the blastoderm and the 'non-wettable'
vitelline surface. During this expansion, the syncytial ring does not lose
its contractility.
IV. Mechanics of Gastrulation
When premorphogenesis comes to an end, the egg has finished acquiring its structure: the separation between cytoplasm and yolk is