96
G.
REVERBERI
FIG. 76. The half anterior vegetal part at the 16-cell, 32-cell and 64-cell stages
respectively.
The map of the 64-cell stage, as proposed by Ortolani (1954, 1955b,
1957b) differs from that proposed by Conklin (1905) and adopted by
von Ubisch, in a point which is absolutely essential: the two A 7.6 cells
are not mesenchymal cells, as Conklin supposed, but chordal cells; the
mesenchymal (and muscular) cells are situated only in the posterior
half of the vegetal hemisphere. For the sake of simplicity let us represent
only the half anterior vegetal part of the 16-, 32- or 64-stages, as in
Fig. 76: in this representation, in the 64-cell stage, the interior ring
consists of six chordal cells; the interior six cells are entodermal. From
this map, it emerges that at the 64-cell stage the chordal cells are 6,
not 4 as Conklin and von Ubisch supposed.
Farinella first tried to establish the minimal quantity of plasms,
contained in the anterior vegetal blastomeres (8-cell stage), which is
necessary for the formation of a brain; secondarily she tried to 'capture'
the plasm responsible for the induction. The experiments consisted in
transplanting under an isolated animal quartet (from an egg at the 8-cell
stage) various derivates of the two anterior vegetal blastomeres, taken
from another egg in the 16- or 32-cell stage; thus, under an animal
quartet, was transplanted either one A 5.1 or one A 5.2 (from another
egg at the 16-cell stage); or one of the blastomeres A 6.2, A 6.4, A 6.3,
in the brief stage between 8-32 cells. In another series of experiments,
von Ubisch destroyed all the presumptive neural cells leaving the chordal,
the mesodermal and the entodermal cells: the embryos formed no brain.
Von Ubisch concluded, again, that chordal, mesodermal and entodermal
cells are deprived of 'inducing' power; otherwise they would induce the
brain from the remaining epidermal ectoderm. As regards these experiments, however, Reverberi and Minganti have demonstrated that the
'evocator' manifests its action only on the presumptive 'neural' ectoderm, not just on any ectoderm.
The entire problem has been, at my suggestion, reconsidered experimentally by Farinella and by Ortolani (in press). Before we describe these
experiments, it is necessary to examine attentively the map of the
Organ-forming' territories at the 64-cell stage. This is the stage when
the different plasms segregate; the cells are purely entodermal, or
neural, or mesodermal, etc.
G.
REVERBERI
FIG. 76. The half anterior vegetal part at the 16-cell, 32-cell and 64-cell stages
respectively.
The map of the 64-cell stage, as proposed by Ortolani (1954, 1955b,
1957b) differs from that proposed by Conklin (1905) and adopted by
von Ubisch, in a point which is absolutely essential: the two A 7.6 cells
are not mesenchymal cells, as Conklin supposed, but chordal cells; the
mesenchymal (and muscular) cells are situated only in the posterior
half of the vegetal hemisphere. For the sake of simplicity let us represent
only the half anterior vegetal part of the 16-, 32- or 64-stages, as in
Fig. 76: in this representation, in the 64-cell stage, the interior ring
consists of six chordal cells; the interior six cells are entodermal. From
this map, it emerges that at the 64-cell stage the chordal cells are 6,
not 4 as Conklin and von Ubisch supposed.
Farinella first tried to establish the minimal quantity of plasms,
contained in the anterior vegetal blastomeres (8-cell stage), which is
necessary for the formation of a brain; secondarily she tried to 'capture'
the plasm responsible for the induction. The experiments consisted in
transplanting under an isolated animal quartet (from an egg at the 8-cell
stage) various derivates of the two anterior vegetal blastomeres, taken
from another egg in the 16- or 32-cell stage; thus, under an animal
quartet, was transplanted either one A 5.1 or one A 5.2 (from another
egg at the 16-cell stage); or one of the blastomeres A 6.2, A 6.4, A 6.3,
in the brief stage between 8-32 cells. In another series of experiments,
von Ubisch destroyed all the presumptive neural cells leaving the chordal,
the mesodermal and the entodermal cells: the embryos formed no brain.
Von Ubisch concluded, again, that chordal, mesodermal and entodermal
cells are deprived of 'inducing' power; otherwise they would induce the
brain from the remaining epidermal ectoderm. As regards these experiments, however, Reverberi and Minganti have demonstrated that the
'evocator' manifests its action only on the presumptive 'neural' ectoderm, not just on any ectoderm.
The entire problem has been, at my suggestion, reconsidered experimentally by Farinella and by Ortolani (in press). Before we describe these
experiments, it is necessary to examine attentively the map of the
Organ-forming' territories at the 64-cell stage. This is the stage when
the different plasms segregate; the cells are purely entodermal, or
neural, or mesodermal, etc.