364
JEAN J. PASTEELS
conditions and by various techniques have shown the amphibian egg to
be a far more suitable material for this purpose.
It will be our aim to review all the facts pertaining to this morphogenetic role of the cortex, and we should like to state now that their
value is general for all Amphibians considered, either Anura or Urodela.
However, if the morphogenetic role of the 'cortex' is proved, Mercer
and Wolpert would nevertheless be correct in their contention that the
concept of the 'cortex' as used by embryologists remains equivocal.
Our own experience with the egg of the clam Barnea (normal egg : see
Pasteels and de Harven, 1962; centrifuged egg, unpublished) is in
entire accordance with that of Mercer and Wolpert on
Psammechinus.
In another review on the cortical changes at fertilization (Pasteels, 1961)
I insisted on the discrepancy between ultrastructure as seen with the
electron microscope, and conclusions which had been drawn from the
studies in vivo. It is true that neither in the sea urchin egg, nor in the
clam egg, can any structure underlying the plasmalemma be seen which
is different from the inner cytoplasm (unless it is the cortical granules or
alveoli of the unfertilized egg). Yolk, mitochondria, and ribosomes are
equally distributed, in direct contact with the plasmalemma or in the
deeper cytoplasm. Nevertheless, good evidence (Chambers, 1917;
Hiramoto, 1957; Mitchison, 1956) has been obtained from the study of
the sea urchin egg and has proved the existence of a gel-like layer,
which is in direct contact and even in continuity with the plasmamembrane with which it acts as a physiological unit. In the case of the
sea urchin, its thickness has been estimated at 1-6 to 5μ. Although no
ultrastructural sign of such a plasmagel could be detected in the egg of
Barnea, the electron microscope nevertheless showed that, at cleavage,
some underlying though undifferentiated cytoplasm accompanies the
stretching plasmalemma into the furrow (Pasteels and de Harven, 1962).
As we shall see, in the case of the amphibian egg, movements or
displacements of the plasma membrane always affect the underlying
cytoplasm in some way, including some of the yolk platelets.
Thus, if the concept of 'cortex' seems difficult to define exactly, we
may tentatively assimilate it to the plasmalemma, plus some underlying
cytoplasm whose thickness and inner limits need to be made precise,
and are probably variable, depending upon the stage or the physiological
state of the egg.
II. Cortex and Bilateral Symmetry
The origin of bilateral symmetry of the amphibian egg is one of the
oldest and most discussed topics of embryology. Many arguments have
been exchanged between preformationists and epigeneticists ; in the
JEAN J. PASTEELS
conditions and by various techniques have shown the amphibian egg to
be a far more suitable material for this purpose.
It will be our aim to review all the facts pertaining to this morphogenetic role of the cortex, and we should like to state now that their
value is general for all Amphibians considered, either Anura or Urodela.
However, if the morphogenetic role of the 'cortex' is proved, Mercer
and Wolpert would nevertheless be correct in their contention that the
concept of the 'cortex' as used by embryologists remains equivocal.
Our own experience with the egg of the clam Barnea (normal egg : see
Pasteels and de Harven, 1962; centrifuged egg, unpublished) is in
entire accordance with that of Mercer and Wolpert on
Psammechinus.
In another review on the cortical changes at fertilization (Pasteels, 1961)
I insisted on the discrepancy between ultrastructure as seen with the
electron microscope, and conclusions which had been drawn from the
studies in vivo. It is true that neither in the sea urchin egg, nor in the
clam egg, can any structure underlying the plasmalemma be seen which
is different from the inner cytoplasm (unless it is the cortical granules or
alveoli of the unfertilized egg). Yolk, mitochondria, and ribosomes are
equally distributed, in direct contact with the plasmalemma or in the
deeper cytoplasm. Nevertheless, good evidence (Chambers, 1917;
Hiramoto, 1957; Mitchison, 1956) has been obtained from the study of
the sea urchin egg and has proved the existence of a gel-like layer,
which is in direct contact and even in continuity with the plasmamembrane with which it acts as a physiological unit. In the case of the
sea urchin, its thickness has been estimated at 1-6 to 5μ. Although no
ultrastructural sign of such a plasmagel could be detected in the egg of
Barnea, the electron microscope nevertheless showed that, at cleavage,
some underlying though undifferentiated cytoplasm accompanies the
stretching plasmalemma into the furrow (Pasteels and de Harven, 1962).
As we shall see, in the case of the amphibian egg, movements or
displacements of the plasma membrane always affect the underlying
cytoplasm in some way, including some of the yolk platelets.
Thus, if the concept of 'cortex' seems difficult to define exactly, we
may tentatively assimilate it to the plasmalemma, plus some underlying
cytoplasm whose thickness and inner limits need to be made precise,
and are probably variable, depending upon the stage or the physiological
state of the egg.
II. Cortex and Bilateral Symmetry
The origin of bilateral symmetry of the amphibian egg is one of the
oldest and most discussed topics of embryology. Many arguments have
been exchanged between preformationists and epigeneticists ; in the