376
JEAN J. PASTEELS
experiments were made on a large scale with eggs of Rana
temporaria
(Pasteels, 1940a, 1948). The results show a real competition
between
points α and β as to the formation of the dorsal lip : both the centre of
the grey crescent (a) and the centre of Born's crescent (β) may have an
influence on the localization of the blastoporal lip. In 1948 we made 170
experiments of this type : in 84 cases the dorsal lip appeared under the
direct influence of the pre-existing grey crescent (position a) ; in 28 cases
the blastopore lip appeared at the centre of Born's crescent (position β) ;
in 38 cases the position of the lip was intermediate between α and β.
It is thus obvious that the cortical field, though rather firm, may be
affected by some drastic conditions. However, we may add that the
cortical field, even when affected, remains 'stronger' than the new
organization brought about by the descent of the yolk along the cortex.
In the 84 cases in which the dorsal lip was directed by the grey crescent
morphogenesis was always normal and led to perfectly symmetrical
embryos. In the other cases, in which the orientation of the dorsal lip
was entirely (28 cases) or partly (38 cases) determined by Born's
crescent, there remained, however, some influence of the former cortical
field: gastrulation went faster on the side corresponding to the grey
crescent, and the embryos obtained were asymmetrical, with all their
organs larger on the side corresponding to the grey crescent (Pig. 8c).
It may thus be concluded that Born's crescent is a rather good
imitation of the grey crescent, not only in its external appearance, but
also as regards its morphogenetic properties. This will lead us to a
comparison between the internal structure of the two types of crescent.
It is clear from Fig. 9 that both structures are remarkably similar.
Figure 9a, shows (more precisely than the diagrams of Fig. 1) the
disposition of the vegetal yolk in a normal symmetrized egg. One
perceives clearly (Fig. 9a') immediately under the plasmalemma a
layer of thick yolk platelets of vegetal type. This is what we have called
the Vitelline wall' (mur vitellin). Now, the same vitelline wall may be
found underneath the plasmalemma of Born's crescent (Fig. 9b).
Two remarks must be made here. First, the similar position of yolk
and cortex in both dorsal structures derives from opposite but convergent
events. In the case of the grey crescent, the ascending cortex has been
dragging along some adhesive yolk ; in the case of Born's crescent, the
descending yolk has been adhering slightly to the cortex. Secondly, if the
'dorsal' structure is remarkably the same, the ventral one is absolutely
different in the two cases (compare the left sides of Fig. 9a and b). In
the normal egg we see a layer of cytoplasm which separates yolk from
cortex, while in the egg which has been turned there is a close contact
between the bulk of the yolk and the cortex.
This may lead to the supposition that the vitelline wall must be of
JEAN J. PASTEELS
experiments were made on a large scale with eggs of Rana
temporaria
(Pasteels, 1940a, 1948). The results show a real competition
between
points α and β as to the formation of the dorsal lip : both the centre of
the grey crescent (a) and the centre of Born's crescent (β) may have an
influence on the localization of the blastoporal lip. In 1948 we made 170
experiments of this type : in 84 cases the dorsal lip appeared under the
direct influence of the pre-existing grey crescent (position a) ; in 28 cases
the blastopore lip appeared at the centre of Born's crescent (position β) ;
in 38 cases the position of the lip was intermediate between α and β.
It is thus obvious that the cortical field, though rather firm, may be
affected by some drastic conditions. However, we may add that the
cortical field, even when affected, remains 'stronger' than the new
organization brought about by the descent of the yolk along the cortex.
In the 84 cases in which the dorsal lip was directed by the grey crescent
morphogenesis was always normal and led to perfectly symmetrical
embryos. In the other cases, in which the orientation of the dorsal lip
was entirely (28 cases) or partly (38 cases) determined by Born's
crescent, there remained, however, some influence of the former cortical
field: gastrulation went faster on the side corresponding to the grey
crescent, and the embryos obtained were asymmetrical, with all their
organs larger on the side corresponding to the grey crescent (Pig. 8c).
It may thus be concluded that Born's crescent is a rather good
imitation of the grey crescent, not only in its external appearance, but
also as regards its morphogenetic properties. This will lead us to a
comparison between the internal structure of the two types of crescent.
It is clear from Fig. 9 that both structures are remarkably similar.
Figure 9a, shows (more precisely than the diagrams of Fig. 1) the
disposition of the vegetal yolk in a normal symmetrized egg. One
perceives clearly (Fig. 9a') immediately under the plasmalemma a
layer of thick yolk platelets of vegetal type. This is what we have called
the Vitelline wall' (mur vitellin). Now, the same vitelline wall may be
found underneath the plasmalemma of Born's crescent (Fig. 9b).
Two remarks must be made here. First, the similar position of yolk
and cortex in both dorsal structures derives from opposite but convergent
events. In the case of the grey crescent, the ascending cortex has been
dragging along some adhesive yolk ; in the case of Born's crescent, the
descending yolk has been adhering slightly to the cortex. Secondly, if the
'dorsal' structure is remarkably the same, the ventral one is absolutely
different in the two cases (compare the left sides of Fig. 9a and b). In
the normal egg we see a layer of cytoplasm which separates yolk from
cortex, while in the egg which has been turned there is a close contact
between the bulk of the yolk and the cortex.
This may lead to the supposition that the vitelline wall must be of