422
CH.
DEVILLERS
One can then try to find out how far the excess amount and the
separation of the yolk account for the peculiarities of egg dynamics
(Devillers, 1956a) and to what extent the latter can be compared to
those of the Amphibian egg.
Beforehand, it must be noted that among Teleosts themselves there
exist quantitative modifications of properties which certainly appear to
j
Ί
.
r ^
χ. cytoplasm volume _
±
, .
depend on variations ot the ratio: ——^
. r rom this point
yolk volume
of view, the egg of Salmo is an extreme term of the series showing certain
differences from the egg of Fundulus, which has relatively less yolk.
These differences would probably be still more considerable with the
eggs of Carassius or oí Perca.
The egg of Teleosts is laid at a structurally younger stage than that of
Amphibia. This is due to the complexity and the duration of the
arranging of the materials, which is not in this case a simple stratification but a process which is mostly active.
Before the first cleavage groove appears, the egg of Amphibia has
acquired its structure: distribution of materials and acquisition of
intrinsic co-ordinates. 'Structuration' and cleavage are two separate
phases. In the egg of Teleosts, polarity is very generally determined at
laying. The arranging of the constituents is incomplete, sometimes only
indicated; it is sometimes not yet finished (Brachydanio) when cleavage
starts. Symmetrization covers a good part of the period of division. The
two phases, 'structuration' and cleavage, interpenetrate.
If we accept the hypothesis of Oppenheimer and Tung, symmetrization sometimes seems to extend very far into premorphogenesis (Salmo).
One must consider this extension of the process of symmetrization as a
direct consequence of the egg structure. The 'crescent' is perhaps formed
very early and the 'total' egg could be symmetrized before cleavage, but
the active portion (blastoderm) is only symmetrized later on, since it
has not yet finished acquiring its structure. This is a consequence of the
progressive shrinking of the cytoplasmic area.
The egg of Acipenser (Fig. 23b) shows us the displacement of the
crescent of symmetrization (light crescent) which becomes superequatorial (Detlaf and Ginsburg, 1954) while it is subequatorial in
Amphibia (Fig. 23a). It seems to remain approximately in this position
in Teleosts where it is localized in the thin periblastic covering. If this is
so, it is situated as in Amphibia (Pasteéis, 1951) in the intermediate zone
where cortex, cytoplasm and yolk are superposed (though the yolk/
cytoplasm relations are reversed). It would be interesting to find out if a
crescent exists in Amia, and in what position.
Of all the gastrular mechanisms, that of epiboly appears to be affected
the most, in comparison with Amphibia, by the egg structure. How-
CH.
DEVILLERS
One can then try to find out how far the excess amount and the
separation of the yolk account for the peculiarities of egg dynamics
(Devillers, 1956a) and to what extent the latter can be compared to
those of the Amphibian egg.
Beforehand, it must be noted that among Teleosts themselves there
exist quantitative modifications of properties which certainly appear to
j
Ί
.
r ^
χ. cytoplasm volume _
±
, .
depend on variations ot the ratio: ——^
. r rom this point
yolk volume
of view, the egg of Salmo is an extreme term of the series showing certain
differences from the egg of Fundulus, which has relatively less yolk.
These differences would probably be still more considerable with the
eggs of Carassius or oí Perca.
The egg of Teleosts is laid at a structurally younger stage than that of
Amphibia. This is due to the complexity and the duration of the
arranging of the materials, which is not in this case a simple stratification but a process which is mostly active.
Before the first cleavage groove appears, the egg of Amphibia has
acquired its structure: distribution of materials and acquisition of
intrinsic co-ordinates. 'Structuration' and cleavage are two separate
phases. In the egg of Teleosts, polarity is very generally determined at
laying. The arranging of the constituents is incomplete, sometimes only
indicated; it is sometimes not yet finished (Brachydanio) when cleavage
starts. Symmetrization covers a good part of the period of division. The
two phases, 'structuration' and cleavage, interpenetrate.
If we accept the hypothesis of Oppenheimer and Tung, symmetrization sometimes seems to extend very far into premorphogenesis (Salmo).
One must consider this extension of the process of symmetrization as a
direct consequence of the egg structure. The 'crescent' is perhaps formed
very early and the 'total' egg could be symmetrized before cleavage, but
the active portion (blastoderm) is only symmetrized later on, since it
has not yet finished acquiring its structure. This is a consequence of the
progressive shrinking of the cytoplasmic area.
The egg of Acipenser (Fig. 23b) shows us the displacement of the
crescent of symmetrization (light crescent) which becomes superequatorial (Detlaf and Ginsburg, 1954) while it is subequatorial in
Amphibia (Fig. 23a). It seems to remain approximately in this position
in Teleosts where it is localized in the thin periblastic covering. If this is
so, it is situated as in Amphibia (Pasteéis, 1951) in the intermediate zone
where cortex, cytoplasm and yolk are superposed (though the yolk/
cytoplasm relations are reversed). It would be interesting to find out if a
crescent exists in Amia, and in what position.
Of all the gastrular mechanisms, that of epiboly appears to be affected
the most, in comparison with Amphibia, by the egg structure. How-