II.
THE EMBRYOLOGY OF ASCIDIANS
97
or A 6.1 (from an egg at the 32-cell stage). The transplanted blastomere
is encircled by the dividing cells of the animal quartet, together with
which it gives rise to a larva. The larvae which develop are, of course,
without muscles and mesenchyme. The larvae which derive from the
transplantation of one A 6.3 show entoderm and notochord; those
which derive from the transplantation of one A 6.4 possess notochord
but no entoderm; those which derive from a transplant of one A 6.2
have also notochord, but no entoderm; and, finally, those which derive
from a transplant of one A 6.1 show entoderm, but no notochord.
In every case a neural organ, with pigmented spots, was obtained. This
means that ^ of the anterior vegetal part of the egg is capable of
inducing. The capacity to induce seems to be a property of all the cells
considered; however it seems stronger in the cells A 6.3 and A 6.4 than
in the cells A 6.1 and A 6.2. The problem as to which plasm, however,
is responsible for the induction does not appear clearly from the
experiments, because, with the exception of A 6.1 (entodermal), the
other cells (32-cell stage) have not yet segregated their plasms.
The experiments of Ortolani consisted in destroying systematically
groups of cells. As the derivatives of the two anterior vegetal blastomeres
contain only 'plasms' for the spinal chord, entoderm, and notochord, it
is evident that the responsibility for the induction is not to be searched
for in other plasms.
(a) In a series of experiments at the 64-cell stage, the 4 spinal chord
cells which form the exterior ring (A 7.8; A 7.4; A 7.8; A 7.4) were
destroyed; as a result, larvae were obtained which were endowed with
normal brain and sensory organs. This result suggests that the inductor
must be searched for in the notochord or in the entodermal cells.
(b) In a second series of experiments (64-cell stage) all ento-dermic
and chordal cells, were destroyed (cf. Fig. 76). The larvae which
developed completely and consistently lacked a brain. Von Ubisch
claims that after destruction of all chordal cells, the tadpoles still
have a brain: but von Ubisch, in fact, did not destroy all the chordal
cells, as the two A 7.6, to which he attributed a mesenchymal fate, were
preserved. From the results of this series of experiments by Ortolani, it
follows that the chordal cells are responsible for the formation of the
brain: the proof is, however, indirect.
(c) In a third series of experiments the 6 entodermal cells (A 7.5;
A 7.1; A 7.2; A 7.5; A 7.1; A 7.2) were destroyed; the tadpoles which
developed had brains. Now, as it was demonstrated that the 4 spinal
cells have no influence on the formation of the brain, one can conclude
(and this is a rather direct demonstration) that the inductor is represented by the chordal cells.
(d) Other types of experiments led to the assumption that the lateral
THE EMBRYOLOGY OF ASCIDIANS
97
or A 6.1 (from an egg at the 32-cell stage). The transplanted blastomere
is encircled by the dividing cells of the animal quartet, together with
which it gives rise to a larva. The larvae which develop are, of course,
without muscles and mesenchyme. The larvae which derive from the
transplantation of one A 6.3 show entoderm and notochord; those
which derive from the transplantation of one A 6.4 possess notochord
but no entoderm; those which derive from a transplant of one A 6.2
have also notochord, but no entoderm; and, finally, those which derive
from a transplant of one A 6.1 show entoderm, but no notochord.
In every case a neural organ, with pigmented spots, was obtained. This
means that ^ of the anterior vegetal part of the egg is capable of
inducing. The capacity to induce seems to be a property of all the cells
considered; however it seems stronger in the cells A 6.3 and A 6.4 than
in the cells A 6.1 and A 6.2. The problem as to which plasm, however,
is responsible for the induction does not appear clearly from the
experiments, because, with the exception of A 6.1 (entodermal), the
other cells (32-cell stage) have not yet segregated their plasms.
The experiments of Ortolani consisted in destroying systematically
groups of cells. As the derivatives of the two anterior vegetal blastomeres
contain only 'plasms' for the spinal chord, entoderm, and notochord, it
is evident that the responsibility for the induction is not to be searched
for in other plasms.
(a) In a series of experiments at the 64-cell stage, the 4 spinal chord
cells which form the exterior ring (A 7.8; A 7.4; A 7.8; A 7.4) were
destroyed; as a result, larvae were obtained which were endowed with
normal brain and sensory organs. This result suggests that the inductor
must be searched for in the notochord or in the entodermal cells.
(b) In a second series of experiments (64-cell stage) all ento-dermic
and chordal cells, were destroyed (cf. Fig. 76). The larvae which
developed completely and consistently lacked a brain. Von Ubisch
claims that after destruction of all chordal cells, the tadpoles still
have a brain: but von Ubisch, in fact, did not destroy all the chordal
cells, as the two A 7.6, to which he attributed a mesenchymal fate, were
preserved. From the results of this series of experiments by Ortolani, it
follows that the chordal cells are responsible for the formation of the
brain: the proof is, however, indirect.
(c) In a third series of experiments the 6 entodermal cells (A 7.5;
A 7.1; A 7.2; A 7.5; A 7.1; A 7.2) were destroyed; the tadpoles which
developed had brains. Now, as it was demonstrated that the 4 spinal
cells have no influence on the formation of the brain, one can conclude
(and this is a rather direct demonstration) that the inductor is represented by the chordal cells.
(d) Other types of experiments led to the assumption that the lateral