CELL DIVISIONS
327
plasm at the blastula stage enables the nuclei to affect the cytoplasm
(Schönmann, 1938).
The experiments on haploid lethal hybrids, as well as those on the
removal and transplantation of nuclei give some evidence of nuclear
activity at the blastula stage. While the above-mentioned diploid
hybrid embryos Triton palmatus $ χ Salamandra
$ die at the late
blastula and during gastrulation, haploid hybrids of the same species
(Triton palmatus (?) χ Salamandra maculosa $) undergo normal cleavage,
but all of them die prior to the onset of gastrulation; anomalies of
mitosis appear in these latter hybrids at the early blastula stage and are
much more various and severe than are those of diploid hybrids. In this
case the conflict between the foreign nucleus and the cytoplasm occurs
at the blastula stage (Boeringer, 1938).
There is also a group of lethal hybrids among anuran hybrids which
undergo development only up to the gastrula (J. Moore, 1941, 1946,
1947, 1948). For example, the diploid hybrids Rana pipiens $ χ Rana
catesbiana $ perish at the beginning of gastrulation (J. Moore, 1941). At
the same time in the androgenetic haploid hybrids Rana pipiens (?) χ
Rana catesbiana
as well as in the androgenetic hybrids Triton
palmatus (?) χ Salamandra maculosa $ the injurious action of the foreign
nucleus reveals itself earlier than in diploid hybrids : their development
is arrested not at the gastrula but at the late blastula (Briggs et al., 1951).
The fact that eggs of R. pipiens inseminated by previously irradiated
sperm of R. catesbiana undergo normal development (Rugh and Exner,
1940) shows that the death of haploid hybrids described above is due to
the activity of a foreign nucleus.
Thus the involvement of the nucleus in development at the blastula
stage, i.e. at the period of asynchronous cleavage, is clearly revealed in
amphibians by the experiments with lethal androgenetic haploid
hybrids. In the presence of the maternal chromosome set (in diploid
hybrids) the injurious action of the paternal chromosome set seems to be
weakened and its effect is manifested later, during gastrulation.
The experiments of Briggs et al. (1951) with enucleated embryos gave
similar results. In the most favourable experimental variant, that is
when Rana pipiens eggs were inseminated with irradiated sperm of
R. catesbiana after the removal of the nucleus, so that they contained
the sperm centriole and non-functioning chromosomes of the male,
the eggs underwent cleavage and formed partial blastulae but never
underwent gastrulation. Embryos with an aneuploid chromosome set
usually behave in a similar way (Dalcq and Simon, 1932; Fankhauser,
1934, 1952).
The moment of nuclear involvement in the development of amphibians
during the period of asynchronous cleavage was for the first time clearly
327
plasm at the blastula stage enables the nuclei to affect the cytoplasm
(Schönmann, 1938).
The experiments on haploid lethal hybrids, as well as those on the
removal and transplantation of nuclei give some evidence of nuclear
activity at the blastula stage. While the above-mentioned diploid
hybrid embryos Triton palmatus $ χ Salamandra
$ die at the late
blastula and during gastrulation, haploid hybrids of the same species
(Triton palmatus (?) χ Salamandra maculosa $) undergo normal cleavage,
but all of them die prior to the onset of gastrulation; anomalies of
mitosis appear in these latter hybrids at the early blastula stage and are
much more various and severe than are those of diploid hybrids. In this
case the conflict between the foreign nucleus and the cytoplasm occurs
at the blastula stage (Boeringer, 1938).
There is also a group of lethal hybrids among anuran hybrids which
undergo development only up to the gastrula (J. Moore, 1941, 1946,
1947, 1948). For example, the diploid hybrids Rana pipiens $ χ Rana
catesbiana $ perish at the beginning of gastrulation (J. Moore, 1941). At
the same time in the androgenetic haploid hybrids Rana pipiens (?) χ
Rana catesbiana
as well as in the androgenetic hybrids Triton
palmatus (?) χ Salamandra maculosa $ the injurious action of the foreign
nucleus reveals itself earlier than in diploid hybrids : their development
is arrested not at the gastrula but at the late blastula (Briggs et al., 1951).
The fact that eggs of R. pipiens inseminated by previously irradiated
sperm of R. catesbiana undergo normal development (Rugh and Exner,
1940) shows that the death of haploid hybrids described above is due to
the activity of a foreign nucleus.
Thus the involvement of the nucleus in development at the blastula
stage, i.e. at the period of asynchronous cleavage, is clearly revealed in
amphibians by the experiments with lethal androgenetic haploid
hybrids. In the presence of the maternal chromosome set (in diploid
hybrids) the injurious action of the paternal chromosome set seems to be
weakened and its effect is manifested later, during gastrulation.
The experiments of Briggs et al. (1951) with enucleated embryos gave
similar results. In the most favourable experimental variant, that is
when Rana pipiens eggs were inseminated with irradiated sperm of
R. catesbiana after the removal of the nucleus, so that they contained
the sperm centriole and non-functioning chromosomes of the male,
the eggs underwent cleavage and formed partial blastulae but never
underwent gastrulation. Embryos with an aneuploid chromosome set
usually behave in a similar way (Dalcq and Simon, 1932; Fankhauser,
1934, 1952).
The moment of nuclear involvement in the development of amphibians
during the period of asynchronous cleavage was for the first time clearly