92
G.
REVERBERI
B. The 'Mosaic* Character of Development
After the first segmentation, the Ascidian egg is no longer an 'equipotential system'; in fact, each blastomere of the 2-cell stage, if isolated,
segments 'partially' and gives rise to a hemi-embryo. At the 2-cell stage
the Ascidian egg is definitely a 'mosaic' egg.
This character has already been shown by Chabry (1887) and Conklin
(1906, 1911) who killed, inside the membranes, one blastomere and
allowed the other to develop. Objections were raised against the method
employed by Chabry and by Conklin; but no different results were
obtained when the eggs were liberated from their membranes, and the
mitochondria are responsible for development, one would expect normal
tadpoles from both fragments. In Almagia's experiments, in fact, both
fragments developed and gastrulated without, however, reaching the
stage of swimming tadpoles. Perhaps better results might be obtained
using precautions which were not taken at the time of these experiments.
The above facts substantiate the view that the unfertilized egg is an
'equipotential system'. This conclusion results also from examining the
structure of the tadpoles which develop from 'dark' fragments. In fact,
they are 'normal' not only in exterior shape, but also in possessing all
structures in the right disposition. It seems, also, that the organs
possess the same number of cells as do the organs of the normal tadpoles.
This is true for the notochord where the cells can be counted in vivo.
The chordal cells in the tadpoles obtained from fragments are 38-40 in
number, as in normal tadpoles, but are, of course, smaller.
This conclusion might be expected after following attentively the
segmentation in the fragments; in fact, they segment in every detail like
a normal egg; the segmentation pattern, the times of segmentation and
the relative proportions between the blastomeres, though on a reduced
scale, are the same as in the normal egg. There is no reason to suppose
that the blastomeres do not divide the same number of times as they do
in the normal egg. In any case, the tadpoles obtained from the fragments
are normal not as a result of compensatory or regenerative processes,
but because every detail of their development is normal. One can in
general assume that when a fragment or an isolated blastomere of an
egg divides in every detail like a normal egg, one must expect from it the
development of a normal organism: this assumption can be considered
valid even if the development does not continue to the end. The
assumption, of course, does not invalidate the opposite assumption,
that a normal organism can be obtained also from a fragment or isolated
blastomere which divides partially (for a discussion, see Weiss, 1939,
p. 297).
G.
REVERBERI
B. The 'Mosaic* Character of Development
After the first segmentation, the Ascidian egg is no longer an 'equipotential system'; in fact, each blastomere of the 2-cell stage, if isolated,
segments 'partially' and gives rise to a hemi-embryo. At the 2-cell stage
the Ascidian egg is definitely a 'mosaic' egg.
This character has already been shown by Chabry (1887) and Conklin
(1906, 1911) who killed, inside the membranes, one blastomere and
allowed the other to develop. Objections were raised against the method
employed by Chabry and by Conklin; but no different results were
obtained when the eggs were liberated from their membranes, and the
mitochondria are responsible for development, one would expect normal
tadpoles from both fragments. In Almagia's experiments, in fact, both
fragments developed and gastrulated without, however, reaching the
stage of swimming tadpoles. Perhaps better results might be obtained
using precautions which were not taken at the time of these experiments.
The above facts substantiate the view that the unfertilized egg is an
'equipotential system'. This conclusion results also from examining the
structure of the tadpoles which develop from 'dark' fragments. In fact,
they are 'normal' not only in exterior shape, but also in possessing all
structures in the right disposition. It seems, also, that the organs
possess the same number of cells as do the organs of the normal tadpoles.
This is true for the notochord where the cells can be counted in vivo.
The chordal cells in the tadpoles obtained from fragments are 38-40 in
number, as in normal tadpoles, but are, of course, smaller.
This conclusion might be expected after following attentively the
segmentation in the fragments; in fact, they segment in every detail like
a normal egg; the segmentation pattern, the times of segmentation and
the relative proportions between the blastomeres, though on a reduced
scale, are the same as in the normal egg. There is no reason to suppose
that the blastomeres do not divide the same number of times as they do
in the normal egg. In any case, the tadpoles obtained from the fragments
are normal not as a result of compensatory or regenerative processes,
but because every detail of their development is normal. One can in
general assume that when a fragment or an isolated blastomere of an
egg divides in every detail like a normal egg, one must expect from it the
development of a normal organism: this assumption can be considered
valid even if the development does not continue to the end. The
assumption, of course, does not invalidate the opposite assumption,
that a normal organism can be obtained also from a fragment or isolated
blastomere which divides partially (for a discussion, see Weiss, 1939,
p. 297).