94
G.
REVERBERI
specimens. He obtained mostly duplicate larvae, but in one case at least
he got a gigantic and normal larva. According to von Ubisch, in the
duplicate larvae, one observes not only a fusion of homologous territories, but also changes in their destiny.
C. The Inductors'
With the progress of development, the plastic powers decrease, and
finally the embryo becomes a rigid 'mosaic' of territories. It is, however,
difficult to determine when a territory has exhausted all its regulative
FIG. 75. Development of the four couples of blastomeres isolated at 8-cell stage
(After Reverberi and Minganti, 1946).
powers, especially because different territories can lose their regulative
powers at different times. It is thus possible to find experimentally that
one territory still keeps its plastic powers, while another develops
according to the laws of a rigid mosaic.
In the Ascidians, as probably elsewhere, one of the most important
moments for establishing whether different territories possess a fixed
destiny or not, is the 8-cell stage. According to Conklin (1905) and
Ortolani (1952, 1954), the presumptive fates of the different blastomeres
at the 8-cell stage is as follows: the two animal anteriors will produce
head epidermis, palps and the brain with its two pigmented spots; the
two animal posteriors will give rise only to epidermis; the two vegetal
anteriors will contain the territory for spinal chord, notochord and a
G.
REVERBERI
specimens. He obtained mostly duplicate larvae, but in one case at least
he got a gigantic and normal larva. According to von Ubisch, in the
duplicate larvae, one observes not only a fusion of homologous territories, but also changes in their destiny.
C. The Inductors'
With the progress of development, the plastic powers decrease, and
finally the embryo becomes a rigid 'mosaic' of territories. It is, however,
difficult to determine when a territory has exhausted all its regulative
FIG. 75. Development of the four couples of blastomeres isolated at 8-cell stage
(After Reverberi and Minganti, 1946).
powers, especially because different territories can lose their regulative
powers at different times. It is thus possible to find experimentally that
one territory still keeps its plastic powers, while another develops
according to the laws of a rigid mosaic.
In the Ascidians, as probably elsewhere, one of the most important
moments for establishing whether different territories possess a fixed
destiny or not, is the 8-cell stage. According to Conklin (1905) and
Ortolani (1952, 1954), the presumptive fates of the different blastomeres
at the 8-cell stage is as follows: the two animal anteriors will produce
head epidermis, palps and the brain with its two pigmented spots; the
two animal posteriors will give rise only to epidermis; the two vegetal
anteriors will contain the territory for spinal chord, notochord and a