330
T. A. DETTLAFF
that enucleated amphibian embryos (Briggs et al., 1951) and sea urchin
embryos (Harvey, 1940) attain the blastula stage but do not start
gastrulation; the androgenetic haploid lethal hybrids Tr.
palmatus
(Ç) χ Salamandra maculosa $ and R. pipiens ($) χ R. catesbiana $ behave
in the same manner (cf. III, A). Experiments on radiation-induced
inactivation of nuclei give still clearer results. Using this method,
Neyfakh (1959, 1961) succeeded in elucidating not only the periods of
morphogenetic activity of the nuclei but also the developmental stages
which predetermined these periods. After inactivation of the nuclei, the
embryos only attained that stage of development up to which the
cytoplasm of their cells had been predetermined by the preceding
activity of the oocyte nucleus. Nuclear inactivation in the period from
fertilization up to the stage of the mid-blastula, i.e. before the onset
of nuclear function, always arrested the development of the embryos of
the loach, sturgeon fishes and several anuran species (Rana
temporaria,
R. esculenta, and R. ribidunda) before the onset of gastrulation. The same
result was obtained in the sea urchin Strongylocentrotus
drobachiensis
after nuclear inactivation from fertilization up to the early blastula
(Neyfakh, 1961). Thus it was convincingly shown that in all these
species the activity of the nucleus and its normal interrelation with the
cytoplasm during the period of asynchronous division is required for the
transition to gastrulation.
C. Morphogenetic Activity of Nuclei and the Number of Cell Divisions
The duration of the period from the onset of nuclear activity up to the
onset of gastrulation is rather significant. When judged by the results of
the experiments with irradiation obtained by Neyfakh (1961), it
takes about J of the whole period from insemination to the onset of
gastrulation in the loach and sturgeon fishes, and about f of this period
in the anuran species studied. It is important that the effect of nuclear
action during this period is cumulative in character: the longer the
period of normal nuclear activity permitted, i.e. the later in this period
the nuclei undergo radiation injury, the later the developmental stage
attained by the irradiated embryos. Since the synthetic function of the
nuclei is mainly realized during the interphase period, it seems likely
that the sum total action of the nuclei during the period of asynchronous
division up to the onset of gastrulation is a function of the number of
interphase states and their relative duration during this period.
As will be shown below, the onset of specific nuclear function in its turn
always takes place at certain cleavage stages. It depends upon the level
of cytoplasmic differentiation in oogenesis and coincides with the attainment of a certain degree of segregation of its components during cleavage.
This is shown most clearly in the so-called 'mosaic' eggs, in which
T. A. DETTLAFF
that enucleated amphibian embryos (Briggs et al., 1951) and sea urchin
embryos (Harvey, 1940) attain the blastula stage but do not start
gastrulation; the androgenetic haploid lethal hybrids Tr.
palmatus
(Ç) χ Salamandra maculosa $ and R. pipiens ($) χ R. catesbiana $ behave
in the same manner (cf. III, A). Experiments on radiation-induced
inactivation of nuclei give still clearer results. Using this method,
Neyfakh (1959, 1961) succeeded in elucidating not only the periods of
morphogenetic activity of the nuclei but also the developmental stages
which predetermined these periods. After inactivation of the nuclei, the
embryos only attained that stage of development up to which the
cytoplasm of their cells had been predetermined by the preceding
activity of the oocyte nucleus. Nuclear inactivation in the period from
fertilization up to the stage of the mid-blastula, i.e. before the onset
of nuclear function, always arrested the development of the embryos of
the loach, sturgeon fishes and several anuran species (Rana
temporaria,
R. esculenta, and R. ribidunda) before the onset of gastrulation. The same
result was obtained in the sea urchin Strongylocentrotus
drobachiensis
after nuclear inactivation from fertilization up to the early blastula
(Neyfakh, 1961). Thus it was convincingly shown that in all these
species the activity of the nucleus and its normal interrelation with the
cytoplasm during the period of asynchronous division is required for the
transition to gastrulation.
C. Morphogenetic Activity of Nuclei and the Number of Cell Divisions
The duration of the period from the onset of nuclear activity up to the
onset of gastrulation is rather significant. When judged by the results of
the experiments with irradiation obtained by Neyfakh (1961), it
takes about J of the whole period from insemination to the onset of
gastrulation in the loach and sturgeon fishes, and about f of this period
in the anuran species studied. It is important that the effect of nuclear
action during this period is cumulative in character: the longer the
period of normal nuclear activity permitted, i.e. the later in this period
the nuclei undergo radiation injury, the later the developmental stage
attained by the irradiated embryos. Since the synthetic function of the
nuclei is mainly realized during the interphase period, it seems likely
that the sum total action of the nuclei during the period of asynchronous
division up to the onset of gastrulation is a function of the number of
interphase states and their relative duration during this period.
As will be shown below, the onset of specific nuclear function in its turn
always takes place at certain cleavage stages. It depends upon the level
of cytoplasmic differentiation in oogenesis and coincides with the attainment of a certain degree of segregation of its components during cleavage.
This is shown most clearly in the so-called 'mosaic' eggs, in which