II.
THE EMBRYOLOGY
OF
ASCIDIANS
95
part of intestine; the two vegetal posteriors will give rise to part of the
intestine, mesenchyme and muscles. This topography of the 'organforming' areas has been established by Conklin, by following the
distribution and segregation of naturally coloured pigments in Styela,
and by Ortolani by the more exact method of marking the blastomeres
with small coloured chalk granules. The result is a map of the 'presumptive' territories which is a bit different from that proposed by
Vandebroek (1938). One is not authorized to deduce from it that the
destiny of any area is fixed; only experiments can give the answer.
The destiny of isolated blastomeres in the egg at the 8-cell stage was
followed by Reverberi and Minganti (1946, 1947): the isolation was done
as indicated in Fig. 75. With great surprise it was observed that, while
all territories (intestine, notochord, muscles, mesenchyme) have their
destiny already fixed, and consequently 'autodifferentiate' according to
their presumptive fate, the destiny of the neural system is not fixed;
the two animal anterior blastomeres, if isolated, do not give rise to
brain or palps or sensory spots but on the contrary, give rise only to
epidermis. Moreover, one obtains from an isolated animal quartet an
ectodermal vesicle, always lacking the brain. As a result of various
other combinations, one arrives at the conclusion that the formation
of the brain is 'dependent', and that the material responsible for its
differentiation is situated in the vegetal anterior blastomeres (Reverberi
and Minganti, 1946). Only by combining the two animal anterior
blastomeres with the two vegetal anterior ones, does one obtain
embryos provided with brains. The two anterior vegetal blastomeres
were, thus, considered as 'responsible' for the formation of the brain.
One might ask if they are 'inductors'.
By other experiments (Reverberi and Minganti, 1947) it was shown
that the vegetal anterior blastomeres do not 'induce' any presumptive
epidermis whatever to become brain; they only act on the 'neural'
ectoderm; they 'evócate' the 'neural' presumptive ectoderm to become
brain; they are simply 'evocators'. It will, however, be seen later what
the real situation is.
Of course, one is immediately confronted with a question: what plasm,
in the anterior vegetal blastomeres, is responsible for the 'evocation'?
The problem was not solved by Reverberi and Minganti. Von Ubisch
(1939) attacked it. Working on Ascidiella aspersa, mainly at the stage
of 32-64 cells, von Ubisch destroyed systematically the mesodermal, or
the chordal, or the entodermal cells. The larvae which developed after
these operations looked, of course, different, but they always had
brains. Von Ubisch concluded that neither mesoderm, nor notochord,
nor entoderm are the inductors of the brain. The inductive stimulus,
proceeding from the anterior vegetal blastomeres, must also work only
THE EMBRYOLOGY
OF
ASCIDIANS
95
part of intestine; the two vegetal posteriors will give rise to part of the
intestine, mesenchyme and muscles. This topography of the 'organforming' areas has been established by Conklin, by following the
distribution and segregation of naturally coloured pigments in Styela,
and by Ortolani by the more exact method of marking the blastomeres
with small coloured chalk granules. The result is a map of the 'presumptive' territories which is a bit different from that proposed by
Vandebroek (1938). One is not authorized to deduce from it that the
destiny of any area is fixed; only experiments can give the answer.
The destiny of isolated blastomeres in the egg at the 8-cell stage was
followed by Reverberi and Minganti (1946, 1947): the isolation was done
as indicated in Fig. 75. With great surprise it was observed that, while
all territories (intestine, notochord, muscles, mesenchyme) have their
destiny already fixed, and consequently 'autodifferentiate' according to
their presumptive fate, the destiny of the neural system is not fixed;
the two animal anterior blastomeres, if isolated, do not give rise to
brain or palps or sensory spots but on the contrary, give rise only to
epidermis. Moreover, one obtains from an isolated animal quartet an
ectodermal vesicle, always lacking the brain. As a result of various
other combinations, one arrives at the conclusion that the formation
of the brain is 'dependent', and that the material responsible for its
differentiation is situated in the vegetal anterior blastomeres (Reverberi
and Minganti, 1946). Only by combining the two animal anterior
blastomeres with the two vegetal anterior ones, does one obtain
embryos provided with brains. The two anterior vegetal blastomeres
were, thus, considered as 'responsible' for the formation of the brain.
One might ask if they are 'inductors'.
By other experiments (Reverberi and Minganti, 1947) it was shown
that the vegetal anterior blastomeres do not 'induce' any presumptive
epidermis whatever to become brain; they only act on the 'neural'
ectoderm; they 'evócate' the 'neural' presumptive ectoderm to become
brain; they are simply 'evocators'. It will, however, be seen later what
the real situation is.
Of course, one is immediately confronted with a question: what plasm,
in the anterior vegetal blastomeres, is responsible for the 'evocation'?
The problem was not solved by Reverberi and Minganti. Von Ubisch
(1939) attacked it. Working on Ascidiella aspersa, mainly at the stage
of 32-64 cells, von Ubisch destroyed systematically the mesodermal, or
the chordal, or the entodermal cells. The larvae which developed after
these operations looked, of course, different, but they always had
brains. Von Ubisch concluded that neither mesoderm, nor notochord,
nor entoderm are the inductors of the brain. The inductive stimulus,
proceeding from the anterior vegetal blastomeres, must also work only