328
T. A. DETTLAFF
and precisely established by Neyfakh (1960; cf. III, Β) in experiments
with radiation-induced damage of nuclei. It was shown by the same
technique that the nuclei in the embryos of Misgurnus fossilis (Neyfakh,
1959a, b) and of Acipenser stellatus (Arman and Neyfakh, 1961) start
functioning at the mid-blastula stage, i.e. during the period of asynchronous cleavage divisions.
III. Morphogenetic Action of Nuclei during the Period of
Asynchronous Cleavage Division
The experiments referred to above not only indicate the presence of
nuclear function at the blastula stage but offer some evidence as to the
possible morphogenetic importance of nuclear activity during this period.
A. Development of Cell Competence and Inducing Capacities
Several data suggest the participation of nuclei in the development of
competence of cells and of their inducing properties. The results of
transplantation of regions of enucleated blastulae of Rana pipiens ($) χ
Rana catesbiana ( c ? ) crosses to early gastrulae or neurulae of diploid
embryos of Rana pipiens are demonstrative in this respect (Briggs
et al., 1951). The transplants took and were preserved for 1-4 days but
no signs of differentiation were observed. The adjacent normal host
tissues did not prolong their viability.
It is of interest that the regions of blastulae taken from certain lethal
hybrids and transplanted in the same way to a normal embryo (for
example, from the lethal diploid hybrid Rana pipiens $ χ Rana sylvatica $
to R. palustris and Ambystoma maculatum (Moore, 1947, 1948); or from
the androgenetic hybrid Triton palmatus (?) χ Triton cristatus $ to the
embryo of Triton palmatus (Hadorn, 1932) not only took and lived for a
long time but differentiated under the influence of the normal host tissues.
When grafted to normal embryos, transplants taken from other lethal
hybrids (e.g., Rana pipiens $ χ Rana catesbiana (?, King and Briggs,
1953 : or from Bufo vulgaris $ χ Rana esculenta
Brächet, 1954) took for
a short time and underwent no differentiation, behaving in the same
way as the transplants from enucleated blastulae.
These experiments (Briggs et al., 1951; King and Briggs, 1953;
Brächet, 1954) are in good agreement with those concerned with
enucleation of some of the blastomeres of sea urchins at the 4 and 8 cell
stages (Lorch et al., 1953). Enucleated blastomeres of embryos of
Paracentrotus
lividus,
Psammechinus
miliaris
and
Echinocardium
cordatum begin cleavage after some delay, divide not quite typically,
and never differentiate ; while remaining within the embryo they do not
participate in the formation of its organs. Moreover, enucleated vegetative blastomeres do not exert the gradient effect upon the differentiation
T. A. DETTLAFF
and precisely established by Neyfakh (1960; cf. III, Β) in experiments
with radiation-induced damage of nuclei. It was shown by the same
technique that the nuclei in the embryos of Misgurnus fossilis (Neyfakh,
1959a, b) and of Acipenser stellatus (Arman and Neyfakh, 1961) start
functioning at the mid-blastula stage, i.e. during the period of asynchronous cleavage divisions.
III. Morphogenetic Action of Nuclei during the Period of
Asynchronous Cleavage Division
The experiments referred to above not only indicate the presence of
nuclear function at the blastula stage but offer some evidence as to the
possible morphogenetic importance of nuclear activity during this period.
A. Development of Cell Competence and Inducing Capacities
Several data suggest the participation of nuclei in the development of
competence of cells and of their inducing properties. The results of
transplantation of regions of enucleated blastulae of Rana pipiens ($) χ
Rana catesbiana ( c ? ) crosses to early gastrulae or neurulae of diploid
embryos of Rana pipiens are demonstrative in this respect (Briggs
et al., 1951). The transplants took and were preserved for 1-4 days but
no signs of differentiation were observed. The adjacent normal host
tissues did not prolong their viability.
It is of interest that the regions of blastulae taken from certain lethal
hybrids and transplanted in the same way to a normal embryo (for
example, from the lethal diploid hybrid Rana pipiens $ χ Rana sylvatica $
to R. palustris and Ambystoma maculatum (Moore, 1947, 1948); or from
the androgenetic hybrid Triton palmatus (?) χ Triton cristatus $ to the
embryo of Triton palmatus (Hadorn, 1932) not only took and lived for a
long time but differentiated under the influence of the normal host tissues.
When grafted to normal embryos, transplants taken from other lethal
hybrids (e.g., Rana pipiens $ χ Rana catesbiana (?, King and Briggs,
1953 : or from Bufo vulgaris $ χ Rana esculenta
Brächet, 1954) took for
a short time and underwent no differentiation, behaving in the same
way as the transplants from enucleated blastulae.
These experiments (Briggs et al., 1951; King and Briggs, 1953;
Brächet, 1954) are in good agreement with those concerned with
enucleation of some of the blastomeres of sea urchins at the 4 and 8 cell
stages (Lorch et al., 1953). Enucleated blastomeres of embryos of
Paracentrotus
lividus,
Psammechinus
miliaris
and
Echinocardium
cordatum begin cleavage after some delay, divide not quite typically,
and never differentiate ; while remaining within the embryo they do not
participate in the formation of its organs. Moreover, enucleated vegetative blastomeres do not exert the gradient effect upon the differentiation