IX. DEVELOPMENT OF THE TELEOSTEAN EGG
385
future blastoderm to which is added a certain amount of peripheral
cytoplasm.
It is convenient to divide bipolar differentiation into two processes:
segregation of the constituents and organization of the blastoderm.
These two processes start in the ovary, but their state at laying varies
(classification of Kowalewsky, 1886).
At laying, in Carassius (Kowalewsky), Siphonostoma (Gudger, 1905),
Alosa (Ryder, 1881), Clupea (Kanoh, 1953) and Brachydanio (RoosenRunge, 1938), segregation is not finished, and organization not even
adumbrated in Brachydanio where it only ends with the first cleavages.
In Leuciscus (Arndt, 1956) or Salmo, segregation is ended. Organization is
already obvious, but it is at a standstill in the maternal organism and it
will only be released by activation. Then, in 4 to 6 hours at 8° C, the
major part of the cytoplasm assembles in the germ and cleavage is about
to begin. The completion of the blastoderm is not a necessary condition
for starting mitosis; the latter can begin in an incomplete germ, either
in natural conditions (Brachydanio, Roosen-Run^e, 1938), or in experimental conditions (organization of the disc of Salmo brought to a standstill by KCN, Devillers et al, 1959).
This first rapid migration during the presegmentation period is
followed during the segmentation period by a slow migration which
seems to end only at the blástula stage in Salmo. What remains of the
periblast joins the germ and perhaps plays a decisive part in symmetrization and determination of the marginal zone (see Section III, B, 2).
1. The Mechanism of Bipolar Differentiation
Segregation might only be due to the physico-chemical state of the egg
components (decrease of surface tension) which would cause the merging
of the yolk droplets (Grodzinski, 1949, 1956).
The mechanism of organization, however, is certainly far more complex and the attempts to explain it are numerous.
(1) The idea of a distribution by gravity of the constituents must be
put aside, for they are not mobile as they are in the egg of Amphibia.
The blastodisc in Salmo appears in any position relative to the vertical.
In the eggs of Alosa or Esox, agitated in the current of a Zug bottle, the
germ develops where it was already marked out on the ripe oocyte.
(2) Spek (1933) invokes an autocataphoresis separating the alkaline
cytoplasmic particles (pH 7*6) from the acid yolk granules (pH 5-6).
This hypothesis has been criticized in different ways: the stains used by
Spek are not pH indicators (Lison, 1935) and direct measurement by
acid indicators show a pH of about 6-8 (Chambers, 1932); the existence
of the process of cataphoresis itself, the origin of which is unexplained,
385
future blastoderm to which is added a certain amount of peripheral
cytoplasm.
It is convenient to divide bipolar differentiation into two processes:
segregation of the constituents and organization of the blastoderm.
These two processes start in the ovary, but their state at laying varies
(classification of Kowalewsky, 1886).
At laying, in Carassius (Kowalewsky), Siphonostoma (Gudger, 1905),
Alosa (Ryder, 1881), Clupea (Kanoh, 1953) and Brachydanio (RoosenRunge, 1938), segregation is not finished, and organization not even
adumbrated in Brachydanio where it only ends with the first cleavages.
In Leuciscus (Arndt, 1956) or Salmo, segregation is ended. Organization is
already obvious, but it is at a standstill in the maternal organism and it
will only be released by activation. Then, in 4 to 6 hours at 8° C, the
major part of the cytoplasm assembles in the germ and cleavage is about
to begin. The completion of the blastoderm is not a necessary condition
for starting mitosis; the latter can begin in an incomplete germ, either
in natural conditions (Brachydanio, Roosen-Run^e, 1938), or in experimental conditions (organization of the disc of Salmo brought to a standstill by KCN, Devillers et al, 1959).
This first rapid migration during the presegmentation period is
followed during the segmentation period by a slow migration which
seems to end only at the blástula stage in Salmo. What remains of the
periblast joins the germ and perhaps plays a decisive part in symmetrization and determination of the marginal zone (see Section III, B, 2).
1. The Mechanism of Bipolar Differentiation
Segregation might only be due to the physico-chemical state of the egg
components (decrease of surface tension) which would cause the merging
of the yolk droplets (Grodzinski, 1949, 1956).
The mechanism of organization, however, is certainly far more complex and the attempts to explain it are numerous.
(1) The idea of a distribution by gravity of the constituents must be
put aside, for they are not mobile as they are in the egg of Amphibia.
The blastodisc in Salmo appears in any position relative to the vertical.
In the eggs of Alosa or Esox, agitated in the current of a Zug bottle, the
germ develops where it was already marked out on the ripe oocyte.
(2) Spek (1933) invokes an autocataphoresis separating the alkaline
cytoplasmic particles (pH 7*6) from the acid yolk granules (pH 5-6).
This hypothesis has been criticized in different ways: the stains used by
Spek are not pH indicators (Lison, 1935) and direct measurement by
acid indicators show a pH of about 6-8 (Chambers, 1932); the existence
of the process of cataphoresis itself, the origin of which is unexplained,