368
JEAN J. PASTEELS
The true mechanism of this cortical reaction remains unexplained : it
may be described as a general movement of the cortical layers on the
inner material.
As may be seen in Fig. 2b, the movement of the cortex is an ascending
one (i.e. towards the animal pole) dorsally, and a descending one
ventrally ; it has an extension of about 30°. By 'cortex' we mean here
not only the plasmalemma, but also the pigment-containing underlying
cytoplasm. This displacement of the cytoplasmic pigmented layer
explains the formation of the grey (or clear) crescent. But it also carries
along, mainly near the vegetal pole, the most superficial part of the
yolk. The 'grey crescent' not only appears as a secondarily depigmented area of the egg, but also as a region where a close contact
occurs between the cortex and the upper part of the vegetal yolk;
while ventrally, cortex and the corresponding part of the yolk remain
separated by a cytoplasmic barrier (Fig. 2b ; see also Fig. 8b).
As we shall see later, the peculiar 'dorsal' structure, corresponding to
a close contact between the cortex and a layer of vegetal yolk, must be
of some morphogenetic significance. This peculiar condition may be
experimentally reproduced and the experimental 'mock grey crescent'
or Born's crescent, has the same qualities for future morphogenetic
events as has the normal one.
There are two final points. ( 1 ) The symmetrization is now fixed. A mere
free rotation of the egg may be repeated again and again without any
effect on the symmetry ; however, as we shall see (Section IV) the drastic
change which is obtained when the egg is placed in a fixed oblique
position may affect it, but only partially. (2) While during the preparatory period some asymmetrical relationship between yolk and cortex
was dependent on displacements of the yolk (the cortex remaining
passive), in the second period of definitive symmetrization, the reverse
situation is realized; i.e. an active displacement of the cortex on the
inner material is here the determining factor of the symmetry.
III. Grey Crescent and 'Cortical Field'
The classical experiment of Schultze (1894) was correctly understood
only when it was repeated more than 30 years later by Penners and
Schleip (1928; see also Penners, 1929). Schultze claimed that the
embryos which he obtained from the two-cell stage in the frog were each
derived from one of the blastomeres. To obtain this result it was
necessary to turn the egg upside down after it had been immobilized.
In fact, as Penners and Schleip demonstrated, it did not matter
whether the egg was segmented or not, and each partner of the doubleembryos did not actually originate from one blastomere. The doubling
JEAN J. PASTEELS
The true mechanism of this cortical reaction remains unexplained : it
may be described as a general movement of the cortical layers on the
inner material.
As may be seen in Fig. 2b, the movement of the cortex is an ascending
one (i.e. towards the animal pole) dorsally, and a descending one
ventrally ; it has an extension of about 30°. By 'cortex' we mean here
not only the plasmalemma, but also the pigment-containing underlying
cytoplasm. This displacement of the cytoplasmic pigmented layer
explains the formation of the grey (or clear) crescent. But it also carries
along, mainly near the vegetal pole, the most superficial part of the
yolk. The 'grey crescent' not only appears as a secondarily depigmented area of the egg, but also as a region where a close contact
occurs between the cortex and the upper part of the vegetal yolk;
while ventrally, cortex and the corresponding part of the yolk remain
separated by a cytoplasmic barrier (Fig. 2b ; see also Fig. 8b).
As we shall see later, the peculiar 'dorsal' structure, corresponding to
a close contact between the cortex and a layer of vegetal yolk, must be
of some morphogenetic significance. This peculiar condition may be
experimentally reproduced and the experimental 'mock grey crescent'
or Born's crescent, has the same qualities for future morphogenetic
events as has the normal one.
There are two final points. ( 1 ) The symmetrization is now fixed. A mere
free rotation of the egg may be repeated again and again without any
effect on the symmetry ; however, as we shall see (Section IV) the drastic
change which is obtained when the egg is placed in a fixed oblique
position may affect it, but only partially. (2) While during the preparatory period some asymmetrical relationship between yolk and cortex
was dependent on displacements of the yolk (the cortex remaining
passive), in the second period of definitive symmetrization, the reverse
situation is realized; i.e. an active displacement of the cortex on the
inner material is here the determining factor of the symmetry.
III. Grey Crescent and 'Cortical Field'
The classical experiment of Schultze (1894) was correctly understood
only when it was repeated more than 30 years later by Penners and
Schleip (1928; see also Penners, 1929). Schultze claimed that the
embryos which he obtained from the two-cell stage in the frog were each
derived from one of the blastomeres. To obtain this result it was
necessary to turn the egg upside down after it had been immobilized.
In fact, as Penners and Schleip demonstrated, it did not matter
whether the egg was segmented or not, and each partner of the doubleembryos did not actually originate from one blastomere. The doubling