378
JEAN J. PASTEELS
some importance for morphogenesis by favouring the interaction
between yolk and (eventually) specialized cortex.
V. Grafting of Cortex Material
A direct demonstration of the morphogenetic significance of the
dorsal cortex has been recently given by Curtis (1960, 1962). Using a
very refined technique this author has been able to excise some cortical
material of the egg (unsegmented or at the very first stages of cleavage)
of Xenopus, and graft it at various sites.
Grafts from grey crescent material, at the time of first cleavage, when
transplanted to the ventral part of another embryo of the same age,
induced the appearance of a secondary axis. Grafts from animal pole or
ventral material when placed in the grey crescent resulted in the failure
of invagination during gastrulation at the graft side, and this causes the
dorsal lip to split in two, which led to the appearance of a double axis
(Curtis, 1960).
Thus the morphogenetic action of the cortex of the grey crescent
region, and the absence of such an action in other parts of the egg such
as the animal pole and the ventral region, have been clearly
demonstrated.
Other experiments, employing the same technique, have allowed
Curtis to explore the evolution of the morphogenetic properties of the
cortex from an unfertilized egg to an 8-cell stage. Cortical material from
unfertilized eggs when grafted into embryos during first cleavage,
inhibits further development; while this unfertilized cortical material,
if grafted into one blastomere of the 2-cell stage, causes a blockage of
cleavage in that half of the embryo which has received the graft (Curtis,
1960). There is thus a definite change in the cortical material after
fertilization. Later (1962), Curtis (cf. Fig. 10a, b) has shown: (1) that
the excision of grey-crescent cortex from embryos at the stage 1 stops
morphogenesis although cleavage continues (permanent blastula) ;
(2) that the same excision of the grey-crescent cortex from an 8-cell
stage results in a normal embryo.
Comparison of these two experiments permits us to conclude that,
between the one- and 8-cell stages, a definite change has occurred in the
dorsal region of the egg. Up to this point we agree completely with
Curtis.
The same evolution is demonstrated in another way (Fig. 10c, d):
the graft of cortical material from the grey crescent of an unsegmented
egg has no effect when placed in the ventral region of an 8-cell stage,
although the reciprocal graft of dorsal cortex of an 8-cell stage into the
ventral part of an unsegmented egg is still effective.
JEAN J. PASTEELS
some importance for morphogenesis by favouring the interaction
between yolk and (eventually) specialized cortex.
V. Grafting of Cortex Material
A direct demonstration of the morphogenetic significance of the
dorsal cortex has been recently given by Curtis (1960, 1962). Using a
very refined technique this author has been able to excise some cortical
material of the egg (unsegmented or at the very first stages of cleavage)
of Xenopus, and graft it at various sites.
Grafts from grey crescent material, at the time of first cleavage, when
transplanted to the ventral part of another embryo of the same age,
induced the appearance of a secondary axis. Grafts from animal pole or
ventral material when placed in the grey crescent resulted in the failure
of invagination during gastrulation at the graft side, and this causes the
dorsal lip to split in two, which led to the appearance of a double axis
(Curtis, 1960).
Thus the morphogenetic action of the cortex of the grey crescent
region, and the absence of such an action in other parts of the egg such
as the animal pole and the ventral region, have been clearly
demonstrated.
Other experiments, employing the same technique, have allowed
Curtis to explore the evolution of the morphogenetic properties of the
cortex from an unfertilized egg to an 8-cell stage. Cortical material from
unfertilized eggs when grafted into embryos during first cleavage,
inhibits further development; while this unfertilized cortical material,
if grafted into one blastomere of the 2-cell stage, causes a blockage of
cleavage in that half of the embryo which has received the graft (Curtis,
1960). There is thus a definite change in the cortical material after
fertilization. Later (1962), Curtis (cf. Fig. 10a, b) has shown: (1) that
the excision of grey-crescent cortex from embryos at the stage 1 stops
morphogenesis although cleavage continues (permanent blastula) ;
(2) that the same excision of the grey-crescent cortex from an 8-cell
stage results in a normal embryo.
Comparison of these two experiments permits us to conclude that,
between the one- and 8-cell stages, a definite change has occurred in the
dorsal region of the egg. Up to this point we agree completely with
Curtis.
The same evolution is demonstrated in another way (Fig. 10c, d):
the graft of cortical material from the grey crescent of an unsegmented
egg has no effect when placed in the ventral region of an 8-cell stage,
although the reciprocal graft of dorsal cortex of an 8-cell stage into the
ventral part of an unsegmented egg is still effective.