EGG CORTEX IN MORPHOGENESIS
383
fourth of the primitive surface (Dollander, 1950, p. 80). The primitive
symmetry is altered (Fig. 12) : in the case of a sagittal ligature, the dorsal
lip of the blastopore — and, of course, the midline of the embryo — is
shifted laterally in such a way that it no longer corresponds to the
centre of the grey crescent but to one of its horns (see Fig. 12). Another
fact which should be considered is that the ligature involves some
change not only of the cortical layer, but also of the inner structure,
viz. the relations between yolk and cytoplasm. A careful analysis of all
his results has lead Dollander to the conclusion that they are in complete
accordance with the theory of Dalcq and Pasteels (1937, 1938), viz. an
interaction between cortical field and inner yolk gradient—but with a
possible modification of the cortical field under the influence of the
stretching.
We have already seen (cf. Born's crescent) that the cortical field can
be considered as a rather fixed structure, but nevertheless subject to
some alteration by drastic experimental procedures.
VII. Cortical Field and Permeability
The statement that stretching could modify the cortical field has lead
Dalcq and Dollander (1948) to the conclusion that the dorsal focus of
this field — i.e. the centre of the grey crescent — could correspond to a
place of increased permeability due to its lesser thickness. In fact,
cytological observation of fixed eggs has led Dalcq and Dollander (1948)
to state that the cortex (plasmalemma plus some underlying material,
see Section I) is thin at the vegetal pole, somewhat thicker at the grey
crescent, but considerably thicker at the animal pole and ventral side.
No differences of this kind can be found until the reaction of
symmetrization.
This idea of a structure of the cortical field depending on differences
in thickness and permeability was further investigated by Dollander
and his co-workers (Dollander and Melnotte, 1952; Dollander, 1956,
1957; Dollander et al., 1959a, b; Dollander and Bonhomme, 1960;
Dollander and Maillet, 1961; see also Dollander, 1961). When living
eggs are placed for some time in a solution of Nile-blue sulphate or
Neutral red, permeability to these dyes may be estimated by the depth
of penetration. In the unfertilized egg of Triton, no significant difference
can be found between any part of the cortex; in the symmetrized
unsegmented egg (i.e. with clear crescent) the extent of the penetration
is negligible ventrally, small at the animal pole, and definitely greater
at the vegetal pole and dorsally. This gradient of permeability (estimated
by the depth of penetration of basic dyes in vivo) appears some time
before the cortical reaction of symmetrization.
383
fourth of the primitive surface (Dollander, 1950, p. 80). The primitive
symmetry is altered (Fig. 12) : in the case of a sagittal ligature, the dorsal
lip of the blastopore — and, of course, the midline of the embryo — is
shifted laterally in such a way that it no longer corresponds to the
centre of the grey crescent but to one of its horns (see Fig. 12). Another
fact which should be considered is that the ligature involves some
change not only of the cortical layer, but also of the inner structure,
viz. the relations between yolk and cytoplasm. A careful analysis of all
his results has lead Dollander to the conclusion that they are in complete
accordance with the theory of Dalcq and Pasteels (1937, 1938), viz. an
interaction between cortical field and inner yolk gradient—but with a
possible modification of the cortical field under the influence of the
stretching.
We have already seen (cf. Born's crescent) that the cortical field can
be considered as a rather fixed structure, but nevertheless subject to
some alteration by drastic experimental procedures.
VII. Cortical Field and Permeability
The statement that stretching could modify the cortical field has lead
Dalcq and Dollander (1948) to the conclusion that the dorsal focus of
this field — i.e. the centre of the grey crescent — could correspond to a
place of increased permeability due to its lesser thickness. In fact,
cytological observation of fixed eggs has led Dalcq and Dollander (1948)
to state that the cortex (plasmalemma plus some underlying material,
see Section I) is thin at the vegetal pole, somewhat thicker at the grey
crescent, but considerably thicker at the animal pole and ventral side.
No differences of this kind can be found until the reaction of
symmetrization.
This idea of a structure of the cortical field depending on differences
in thickness and permeability was further investigated by Dollander
and his co-workers (Dollander and Melnotte, 1952; Dollander, 1956,
1957; Dollander et al., 1959a, b; Dollander and Bonhomme, 1960;
Dollander and Maillet, 1961; see also Dollander, 1961). When living
eggs are placed for some time in a solution of Nile-blue sulphate or
Neutral red, permeability to these dyes may be estimated by the depth
of penetration. In the unfertilized egg of Triton, no significant difference
can be found between any part of the cortex; in the symmetrized
unsegmented egg (i.e. with clear crescent) the extent of the penetration
is negligible ventrally, small at the animal pole, and definitely greater
at the vegetal pole and dorsally. This gradient of permeability (estimated
by the depth of penetration of basic dyes in vivo) appears some time
before the cortical reaction of symmetrization.