30
J . B. GURDON
ters in Amoeba by replicating cytoplasmic components has not yet been
conclusively demonstrated.
2. Amphibia
Embryos resulting from nuclear transfers between different species of
frogs are usually arrested in development before any distinguishing
characters have been differentiated (Gurdon, 1962c). However, viable
and fertile individuals can be obtained from nuclear transfers between
subspecies. Reciprocal nuclear transplant frogs from Xenopus laevis
laevis and Xenopus laevis victorianus are like the nuclear subspecies in
all larval and adult characters, except for those that can be attributed
to persistent properties of the egg cytoplasm such as the size and coloration of early embryos (Gurdon, 1961). Sambuichi (1961) found that
reciprocal nuclear transplant hybrids between Rana nigromaculata nigromaculata and Rana migromaculata brevipoda showed some influence of
the cytoplasmic species in respect of leucophore distribution in tadpoles
and colour pattern of young frogs. However this effect was observed in
only a few of the nuclear transplants, and the majority were like the
nuclear subspecies in all characters. The general conclusion from these
experiments is that most of the characters of such nuclear transplant
hybrids are under the control of the nucleus and not the cytoplasm.
3. Stentor
By means of the manipulations outlined in Section II, Tartar (1961)
has been able to graft a chain of nuclear nodes from one species of
Stentor into the enucleated cytoplasm of another species. As in other
organisms, the extent to which nuclei and cytoplasm from different
species can develop is broadly related to the genetic difference between
the species. Unfortunately it is not possible to determine to what extent
the nucleus of one species determines differentiation in the cytoplasm
of another species, since the structural characters of the Stentor species
in which heterotransfers are viable are not sufficiently different to
distinguish clearly nuclear from cytoplasmic influences.
4. Conclusion
Nuclear transfer experiments have not yet provided conclusive
evidence of self-replicating cytoplasmic components. The differentiation of some characters (mostly in Amoeba) shows an influence of the
cytoplasm. This could be due to replicating cytoplasmic components,
but can also be explained in some cases by the persistence of cytoplasmic substances (such as yolk), and in other cases by the control of
gene expression by the cytoplasm.
J . B. GURDON
ters in Amoeba by replicating cytoplasmic components has not yet been
conclusively demonstrated.
2. Amphibia
Embryos resulting from nuclear transfers between different species of
frogs are usually arrested in development before any distinguishing
characters have been differentiated (Gurdon, 1962c). However, viable
and fertile individuals can be obtained from nuclear transfers between
subspecies. Reciprocal nuclear transplant frogs from Xenopus laevis
laevis and Xenopus laevis victorianus are like the nuclear subspecies in
all larval and adult characters, except for those that can be attributed
to persistent properties of the egg cytoplasm such as the size and coloration of early embryos (Gurdon, 1961). Sambuichi (1961) found that
reciprocal nuclear transplant hybrids between Rana nigromaculata nigromaculata and Rana migromaculata brevipoda showed some influence of
the cytoplasmic species in respect of leucophore distribution in tadpoles
and colour pattern of young frogs. However this effect was observed in
only a few of the nuclear transplants, and the majority were like the
nuclear subspecies in all characters. The general conclusion from these
experiments is that most of the characters of such nuclear transplant
hybrids are under the control of the nucleus and not the cytoplasm.
3. Stentor
By means of the manipulations outlined in Section II, Tartar (1961)
has been able to graft a chain of nuclear nodes from one species of
Stentor into the enucleated cytoplasm of another species. As in other
organisms, the extent to which nuclei and cytoplasm from different
species can develop is broadly related to the genetic difference between
the species. Unfortunately it is not possible to determine to what extent
the nucleus of one species determines differentiation in the cytoplasm
of another species, since the structural characters of the Stentor species
in which heterotransfers are viable are not sufficiently different to
distinguish clearly nuclear from cytoplasmic influences.
4. Conclusion
Nuclear transfer experiments have not yet provided conclusive
evidence of self-replicating cytoplasmic components. The differentiation of some characters (mostly in Amoeba) shows an influence of the
cytoplasm. This could be due to replicating cytoplasmic components,
but can also be explained in some cases by the persistence of cytoplasmic substances (such as yolk), and in other cases by the control of
gene expression by the cytoplasm.