EGG CORTEX IN MORPHOGENESIS
371
to the formation of the dorsal axis of the embryo, could arise in contact
with each area of white yolk, either at the (primary) vegetal, or at the
(primary) animal pole. Moreover, they perceived some influence from
the region of the grey crescent, which had been marked before the
experiment. They therefore came to the conclusion that : 'Die Lokalisation der ersten Urmundanlage ist determiniert durch die besonderen
Fähigkeiten der Region des grauen Halbmondes oder durch den Einfluss
hellweissen Materiales auf anstossenden dunkleres oder durch beide
Momente gemeinsam' (Penners and Schleip, 1928, p. 404). This conclusion is, however, too cautious to be entirely satisfactory.
Unhappily, the fundamental work of Penners and Schleip (published
in 1928) remained under the shadow of Spemann's Organizationszentrum', still in its full bloom, and was, if not neglected, entirely misunderstood. Being aware of the great importance of the facts discovered
by the German authors, but also of the necessity to come to a clearer
understanding of the respective roles of yolk and
£
grey crescent' factors,
I repeated their experiments. I was able to conclude (Pasteels, 1938b,
1939): (1) that in every case, the position of the dorsal lip (and
embryo-anläge) is determined by an interaction of both factors, i.e. the
occurrence of a white yolk area and some influence from the grey
crescent area; (2) that the latter influence could be better understood if
we admitted that it originated not from the whole crescent, but only
from its central point. This is illustrated by the schematic drawings of
Fig. 4 and by two examples taken from my protocols, in Figs. 5 and 6.
FIG. 5. Inverted egg of Rana temporaria at gastrulation; seen (left) at the vegetal
pole, and (right) at the animal pole. Asterisks indicate the central point of the greycrescent noted before the experiment. The two yolk areas being large, one single blastoporal lip appears at each pole at the border of the vegetal yolk which is nearest to the
centre of the grey crescent.
Dorsal blastoporal lips, leading to the formation of a dorsal embryoanlage, may appear at any point of the egg surface where the contact of
the cortex and yolk margins is the nearest to the centre of the previously
marked grey crescent. These areas of vegetal yolk may be in contact
primarily with the cortex of the vegetal pole, or secondarily with the
371
to the formation of the dorsal axis of the embryo, could arise in contact
with each area of white yolk, either at the (primary) vegetal, or at the
(primary) animal pole. Moreover, they perceived some influence from
the region of the grey crescent, which had been marked before the
experiment. They therefore came to the conclusion that : 'Die Lokalisation der ersten Urmundanlage ist determiniert durch die besonderen
Fähigkeiten der Region des grauen Halbmondes oder durch den Einfluss
hellweissen Materiales auf anstossenden dunkleres oder durch beide
Momente gemeinsam' (Penners and Schleip, 1928, p. 404). This conclusion is, however, too cautious to be entirely satisfactory.
Unhappily, the fundamental work of Penners and Schleip (published
in 1928) remained under the shadow of Spemann's Organizationszentrum', still in its full bloom, and was, if not neglected, entirely misunderstood. Being aware of the great importance of the facts discovered
by the German authors, but also of the necessity to come to a clearer
understanding of the respective roles of yolk and
£
grey crescent' factors,
I repeated their experiments. I was able to conclude (Pasteels, 1938b,
1939): (1) that in every case, the position of the dorsal lip (and
embryo-anläge) is determined by an interaction of both factors, i.e. the
occurrence of a white yolk area and some influence from the grey
crescent area; (2) that the latter influence could be better understood if
we admitted that it originated not from the whole crescent, but only
from its central point. This is illustrated by the schematic drawings of
Fig. 4 and by two examples taken from my protocols, in Figs. 5 and 6.
FIG. 5. Inverted egg of Rana temporaria at gastrulation; seen (left) at the vegetal
pole, and (right) at the animal pole. Asterisks indicate the central point of the greycrescent noted before the experiment. The two yolk areas being large, one single blastoporal lip appears at each pole at the border of the vegetal yolk which is nearest to the
centre of the grey crescent.
Dorsal blastoporal lips, leading to the formation of a dorsal embryoanlage, may appear at any point of the egg surface where the contact of
the cortex and yolk margins is the nearest to the centre of the previously
marked grey crescent. These areas of vegetal yolk may be in contact
primarily with the cortex of the vegetal pole, or secondarily with the