374
JEAN J. PASTEELS
ments, the clear crescent region has been swept by flowing yolk, or
completely covered by this yolk. Nevertheless, the influence of the
'clear crescent' is unaffected : this implies that that influence must be
considered as originating from the cortex and not from an inner plasm
as Lehmann (1945) had stated. Again, in these experiments, we see that
this cortical influence may express itself as far as the immediate vicinity
of animal or vegetal pole.
These facts, and also all the results from the inverted eggs, have led
me (Pasteels, 1938b) to the hypothesis of a cortical field, with its focus at
the dorsal equatorial side and a general decrease towards the ventral
side. In this hypothesis, morphogenesis is supposed to be initiated by an
interaction between the field and the internal yolk gradient. This has
been further elaborated in the general theory of the 'potentiel morphogénétique' by Dalcq and Pasteels (1937 and 1938 ; see also Dalcq, 1941 ;
Pasteels, 1953). As we have seen above, prior to the appearance of the
clear crescent, the cortex does not affect symmetry ('preparation'
towards symmetry resulting always from minute, and reversible,
internal changes). The cortical field is a result of the reorganization of
the cortex, resulting from the drastic events which lead to the clear
crescent ('cortical reaction of symmetrization'). The cortex has not only
acquired a field-like organization, but this organization seems to be
firmly fixed, since in most cases it resists extensive changes of relations
between inner yolk and cytoplasm. However, the cortical field is not
entirely fixed and may respond to some experimental conditions which
we will now consider.
IV. Born's Crescent, or an Artificial 'Grey Crescent*
It has been known since the experiments of Born (1885) that, in the
egg of Rana temporaria, an 'artificial' grey crescent can be created, with
both the aspect, and in some way the effects, of the normal one.
When eggs are unable to rotate freely inside their membranes and are
placed in an oblique position, the heavy yolk, affected by gravity, flows
along the cortex towards the bottom of the egg. To consider the
morphogenetic results of such an experiment we must distinguish among
several cases. (1) The egg is placed in a forced oblique position before
the reaction of symmetrization occurs; (2) the egg is placed in this
position after symmetrization, but the rotation occurs around an axis
parallel to the plane of bilateral symmetry (i.e. the yolk flows in a
direction perpendicular to the bilateral symmetry as established by the
grey crescent); (3) the rotation occurs also after symmetrization but
around an axis perpendicular to the plane of symmetry (i.e. the yolk
flows along the same direction as the pre-existing bilateral symmetry).
JEAN J. PASTEELS
ments, the clear crescent region has been swept by flowing yolk, or
completely covered by this yolk. Nevertheless, the influence of the
'clear crescent' is unaffected : this implies that that influence must be
considered as originating from the cortex and not from an inner plasm
as Lehmann (1945) had stated. Again, in these experiments, we see that
this cortical influence may express itself as far as the immediate vicinity
of animal or vegetal pole.
These facts, and also all the results from the inverted eggs, have led
me (Pasteels, 1938b) to the hypothesis of a cortical field, with its focus at
the dorsal equatorial side and a general decrease towards the ventral
side. In this hypothesis, morphogenesis is supposed to be initiated by an
interaction between the field and the internal yolk gradient. This has
been further elaborated in the general theory of the 'potentiel morphogénétique' by Dalcq and Pasteels (1937 and 1938 ; see also Dalcq, 1941 ;
Pasteels, 1953). As we have seen above, prior to the appearance of the
clear crescent, the cortex does not affect symmetry ('preparation'
towards symmetry resulting always from minute, and reversible,
internal changes). The cortical field is a result of the reorganization of
the cortex, resulting from the drastic events which lead to the clear
crescent ('cortical reaction of symmetrization'). The cortex has not only
acquired a field-like organization, but this organization seems to be
firmly fixed, since in most cases it resists extensive changes of relations
between inner yolk and cytoplasm. However, the cortical field is not
entirely fixed and may respond to some experimental conditions which
we will now consider.
IV. Born's Crescent, or an Artificial 'Grey Crescent*
It has been known since the experiments of Born (1885) that, in the
egg of Rana temporaria, an 'artificial' grey crescent can be created, with
both the aspect, and in some way the effects, of the normal one.
When eggs are unable to rotate freely inside their membranes and are
placed in an oblique position, the heavy yolk, affected by gravity, flows
along the cortex towards the bottom of the egg. To consider the
morphogenetic results of such an experiment we must distinguish among
several cases. (1) The egg is placed in a forced oblique position before
the reaction of symmetrization occurs; (2) the egg is placed in this
position after symmetrization, but the rotation occurs around an axis
parallel to the plane of bilateral symmetry (i.e. the yolk flows in a
direction perpendicular to the bilateral symmetry as established by the
grey crescent); (3) the rotation occurs also after symmetrization but
around an axis perpendicular to the plane of symmetry (i.e. the yolk
flows along the same direction as the pre-existing bilateral symmetry).