II. THE EMBRYOLOGY OF
ASCIDIANS
81
the foreign sperm does not influence these first manifestations of
development. The presence of the anomalous nucleus is, however, felt
at the moment of gastrulation, when the morphogenetic movements
appear, and the protein synthesis characteristic of differentiation begins.
The chemical interactions between nucleus and cytoplasm are no longer
possible and a major process of (paternal) chromatin elimination begins.
The foreign nucleus is not expelled 'in toto', but gradually, and only
through the mitotic process. Clumps of chromatin can be seen, in pycnotic form, outside the spindle; they do not participate in the reconstruction of the nuclei, but are eliminated in masses. In consequence
of this elimination, the cells become haploid; perhaps the elimination
is a device to escape catastrophe, but as this cannot be avoided the
embryos die. One can be sure that death is not a consequence of the
haploid condition, for the experiments of merogony show that an anucleate fragment of the egg can develop, if fertilized, into a normal
larva. Perhaps death is a consequence of an unbalanced condition
between cells which have reached the haploid state and cells which have
not yet eliminated the foreign chromatin. Possibly the problem could
be solved by transplanting a group of cells on to healthy embryos.
But the main question, not restricted, of course, to the lethal hybrids
of Ascidians is this: is the collaboration between the abnormal nucleus
and the cytoplasm broken at gastrulation? In the Amphibians, according
to Brächet (1954), Zeller (1956) and Moore (1957), the cytoplasm of the
hybrid cells is no longer capable at gastrulation of utilizing the RNA
synthesized in the abnormal nucleus; possibly the same is also true in
the Ascidians, but here chemical research appears more difficult than
in the Amphibians, and the only way to elucidate the problem is to
employ cytochemical reactions.
(b) Partially viable hybrids are those which pass gastrulation, but do
not reach the stage of swimming larvae. The compatibility between the
abnormal nucleus and the cytoplasm is apparently greater than in the
lethal hybrids. The cells of these partially viable hybrids are generally
haploid: the paternal chromatin is eliminated. Only in the cross of
Phallusia mamillata ? χ Ascidia méntula
(c) Viable hybrids. These are represented by normally developed and
differentiated larvae (Figs. 24 to 29, Ascidia méntula ? χ Phallusia
mamillata <3\ Figs. 30 to 35, Ascidia malaca ? χ Phallusia mamillata cJ).
The results are not the same in reciprocal crosses: e.g. while the hybrids
of A. malaca $ χ Phallusia 3 reach the stage of swimming larva, the
hybrids of the reciprocal cross (Phallusia %xA. malaca <$) do not
survive much beyond gastrulation (partially viable). The velocity of
development is not influenced by the foreign sperm (Fig. 31 in
ASCIDIANS
81
the foreign sperm does not influence these first manifestations of
development. The presence of the anomalous nucleus is, however, felt
at the moment of gastrulation, when the morphogenetic movements
appear, and the protein synthesis characteristic of differentiation begins.
The chemical interactions between nucleus and cytoplasm are no longer
possible and a major process of (paternal) chromatin elimination begins.
The foreign nucleus is not expelled 'in toto', but gradually, and only
through the mitotic process. Clumps of chromatin can be seen, in pycnotic form, outside the spindle; they do not participate in the reconstruction of the nuclei, but are eliminated in masses. In consequence
of this elimination, the cells become haploid; perhaps the elimination
is a device to escape catastrophe, but as this cannot be avoided the
embryos die. One can be sure that death is not a consequence of the
haploid condition, for the experiments of merogony show that an anucleate fragment of the egg can develop, if fertilized, into a normal
larva. Perhaps death is a consequence of an unbalanced condition
between cells which have reached the haploid state and cells which have
not yet eliminated the foreign chromatin. Possibly the problem could
be solved by transplanting a group of cells on to healthy embryos.
But the main question, not restricted, of course, to the lethal hybrids
of Ascidians is this: is the collaboration between the abnormal nucleus
and the cytoplasm broken at gastrulation? In the Amphibians, according
to Brächet (1954), Zeller (1956) and Moore (1957), the cytoplasm of the
hybrid cells is no longer capable at gastrulation of utilizing the RNA
synthesized in the abnormal nucleus; possibly the same is also true in
the Ascidians, but here chemical research appears more difficult than
in the Amphibians, and the only way to elucidate the problem is to
employ cytochemical reactions.
(b) Partially viable hybrids are those which pass gastrulation, but do
not reach the stage of swimming larvae. The compatibility between the
abnormal nucleus and the cytoplasm is apparently greater than in the
lethal hybrids. The cells of these partially viable hybrids are generally
haploid: the paternal chromatin is eliminated. Only in the cross of
Phallusia mamillata ? χ Ascidia méntula
(c) Viable hybrids. These are represented by normally developed and
differentiated larvae (Figs. 24 to 29, Ascidia méntula ? χ Phallusia
mamillata <3\ Figs. 30 to 35, Ascidia malaca ? χ Phallusia mamillata cJ).
The results are not the same in reciprocal crosses: e.g. while the hybrids
of A. malaca $ χ Phallusia 3 reach the stage of swimming larva, the
hybrids of the reciprocal cross (Phallusia %xA. malaca <$) do not
survive much beyond gastrulation (partially viable). The velocity of
development is not influenced by the foreign sperm (Fig. 31 in