98
G.
REVERBERI
(A 7.6; A 7.6) and the nitermediate (A 7.7; A 7.7) chordal cells are, in
normal morphogenesis, the actual inductors; the medial (A 7.3; A 7.3)
chordal cells are not implicated in the induction. This can easily be
explained by the fact that the medial chordal cells, as shown by the
method of coloured chalk granules, move caudally after the gastrulation and, without making any contact with the presumptive 'neural'
ectoderm, arrive at the tip of the tail. The contact with the 'neural'
ectoderm is, however, established by the intermediate (A 7.7; A 7.7) and
particularly, by the lateral (A 7.6; A 7.6) chordal cells (i.e. their derivatives, of course).
(e) The above data show that the induction of the brain is not due to
a transient influence, coming from the anterior vegetal blastomeres of
the 8-cell stage, as first supposed, in accordance with the results obtained by von Ubisch, but it is the result of a contact between the
chordal cells and the overlying 'neural' ectoderm.
(/) Other experiments show, moreover, that the entodermal cells
A 6.1; A 6.1 (stage 32-cells); or A 7.2, A 7.2 (64-cell stage) are also
capable of inducing. In fact, in a neurula the derivates of A 6.1 are to be
found under the neural plate at the most anterior part of the embryo:
they co-operate with the chordal cells in the induction of a normal
neural plate.
In conclusion, one can affirm that, in the Ascidians, the destiny of the
presumptive 'neural' ectoderm is not fixed at the 8-cell stage, at which
stage all the other territories are already 'determined'. The presumptive
'neural' ectoderm is converted into the neural system only under the
inductive action of the immediately underlying chordal (and entodermic) cells. The induction is consequent on a contact between the
correlated territories. The formation of the neural system in the
Ascidians is then strictly directed by the same laws which Spemann
discovered in the Amphibians. Only slight differences are to be noticed
between the inductive system in the Amphibians and in the Ascidians:
in the Amphibians, the inductor is the chorda-mesoderm, in the Ascidians it is (part of) the chorda-entoderm. Another difference is that the
'ventral' ectoderm in the Amphibians can be induced to form a brain,
and that in the Ascidians, on the contrary, it cannot.
On examining in vivo an Ascidian neurula, Clavelina for example, one
is profoundly impressed by its strict resemblance to a neurula of the
Amphibians. Really, there should be no reason why two such similar
forms should not have the same mechanism responsible for their
formation!
G.
REVERBERI
(A 7.6; A 7.6) and the nitermediate (A 7.7; A 7.7) chordal cells are, in
normal morphogenesis, the actual inductors; the medial (A 7.3; A 7.3)
chordal cells are not implicated in the induction. This can easily be
explained by the fact that the medial chordal cells, as shown by the
method of coloured chalk granules, move caudally after the gastrulation and, without making any contact with the presumptive 'neural'
ectoderm, arrive at the tip of the tail. The contact with the 'neural'
ectoderm is, however, established by the intermediate (A 7.7; A 7.7) and
particularly, by the lateral (A 7.6; A 7.6) chordal cells (i.e. their derivatives, of course).
(e) The above data show that the induction of the brain is not due to
a transient influence, coming from the anterior vegetal blastomeres of
the 8-cell stage, as first supposed, in accordance with the results obtained by von Ubisch, but it is the result of a contact between the
chordal cells and the overlying 'neural' ectoderm.
(/) Other experiments show, moreover, that the entodermal cells
A 6.1; A 6.1 (stage 32-cells); or A 7.2, A 7.2 (64-cell stage) are also
capable of inducing. In fact, in a neurula the derivates of A 6.1 are to be
found under the neural plate at the most anterior part of the embryo:
they co-operate with the chordal cells in the induction of a normal
neural plate.
In conclusion, one can affirm that, in the Ascidians, the destiny of the
presumptive 'neural' ectoderm is not fixed at the 8-cell stage, at which
stage all the other territories are already 'determined'. The presumptive
'neural' ectoderm is converted into the neural system only under the
inductive action of the immediately underlying chordal (and entodermic) cells. The induction is consequent on a contact between the
correlated territories. The formation of the neural system in the
Ascidians is then strictly directed by the same laws which Spemann
discovered in the Amphibians. Only slight differences are to be noticed
between the inductive system in the Amphibians and in the Ascidians:
in the Amphibians, the inductor is the chorda-mesoderm, in the Ascidians it is (part of) the chorda-entoderm. Another difference is that the
'ventral' ectoderm in the Amphibians can be induced to form a brain,
and that in the Ascidians, on the contrary, it cannot.
On examining in vivo an Ascidian neurula, Clavelina for example, one
is profoundly impressed by its strict resemblance to a neurula of the
Amphibians. Really, there should be no reason why two such similar
forms should not have the same mechanism responsible for their
formation!