334
T. A. DETTLAFF
Briggs, 1947). A larger or smaller size of cell, compared to the diploid
control, is accompanied by a corresponding difference in the total
number of cells at equivalent developmental stages, so that the size of
the embryo at different degrees of ploidy remains unchanged. The
regulation of the nucleo-cytoplasmic relations and of cell number in the
embryo are processes closely interrelated but apparently not identical.
It may be recalled (cf. II) that regulation of the volumes of nucleus
and cytoplasm also proceeds during the development of normal diploid
embryos. As to the regulation of cell numbers and the constancy of the
size of the embryo, the mechanism of these processes, according to
Fankhauser (1952) and Briggs and King (1959), still remains obscure.
Let us try to approach this problem from the viewpoint of the
relationship of the processes of cleavage and gastrulation. If the onset of
gastrulation is determined by the activity of the nuclei in their interrelation with the cytoplasm and, on the other hand, gastrulation under
normal conditions begins after a certain number of cleavage divisions
(IV, A), then it can be suggested that the onset of gastrulation is
determined by an accumulation of a certain threshold amount of some
substances bringing about a prerequisite change in the state of the
cytoplasm.
If this is so, the time of the formation of this 'threshold' amount must
be a function of the ploidy, on the one hand, and of the number and
duration of interphases during which this amount can be produced, on
the other. It is natural to suppose that, when ploidy is increased, the
same amount of the products of active nuclei will be obtained from a
smaller number of interphases after the start of nuclear functioning,
while with lower ploidy a greater number of interphases will be required.
However, another interpretation of these facts is also likely. The
possibility is not ruled out that the normal nucleo-cytoplasmic relationship is established in polyploids somewhat earlier than in diploids, and
correspondingly, the morphogenetic nuclear function and gastrulation
determined by this set in at earlier developmental stages, while in
haploids these processes are shifted to later cleavage stages.
At the same time, one should bear in mind that between the production by the nuclei of the threshold amount of substances necessary for
the onset of gastrulation and the onset itself a certain time must elapse
which is required for a change in the state of the cytoplasm. The
experiments on radiation-induced damage of nuclei (Neyfakh, 1961)
have shown in all the animals studied that several hours elapse between
the period during which nuclear activity provides for the onset of
gastrulation and the realization of this action. It can be supposed that
after the formation of the same threshold amount of the products of
nuclear activity per unit of cytoplasmic volume, in eggs of different
T. A. DETTLAFF
Briggs, 1947). A larger or smaller size of cell, compared to the diploid
control, is accompanied by a corresponding difference in the total
number of cells at equivalent developmental stages, so that the size of
the embryo at different degrees of ploidy remains unchanged. The
regulation of the nucleo-cytoplasmic relations and of cell number in the
embryo are processes closely interrelated but apparently not identical.
It may be recalled (cf. II) that regulation of the volumes of nucleus
and cytoplasm also proceeds during the development of normal diploid
embryos. As to the regulation of cell numbers and the constancy of the
size of the embryo, the mechanism of these processes, according to
Fankhauser (1952) and Briggs and King (1959), still remains obscure.
Let us try to approach this problem from the viewpoint of the
relationship of the processes of cleavage and gastrulation. If the onset of
gastrulation is determined by the activity of the nuclei in their interrelation with the cytoplasm and, on the other hand, gastrulation under
normal conditions begins after a certain number of cleavage divisions
(IV, A), then it can be suggested that the onset of gastrulation is
determined by an accumulation of a certain threshold amount of some
substances bringing about a prerequisite change in the state of the
cytoplasm.
If this is so, the time of the formation of this 'threshold' amount must
be a function of the ploidy, on the one hand, and of the number and
duration of interphases during which this amount can be produced, on
the other. It is natural to suppose that, when ploidy is increased, the
same amount of the products of active nuclei will be obtained from a
smaller number of interphases after the start of nuclear functioning,
while with lower ploidy a greater number of interphases will be required.
However, another interpretation of these facts is also likely. The
possibility is not ruled out that the normal nucleo-cytoplasmic relationship is established in polyploids somewhat earlier than in diploids, and
correspondingly, the morphogenetic nuclear function and gastrulation
determined by this set in at earlier developmental stages, while in
haploids these processes are shifted to later cleavage stages.
At the same time, one should bear in mind that between the production by the nuclei of the threshold amount of substances necessary for
the onset of gastrulation and the onset itself a certain time must elapse
which is required for a change in the state of the cytoplasm. The
experiments on radiation-induced damage of nuclei (Neyfakh, 1961)
have shown in all the animals studied that several hours elapse between
the period during which nuclear activity provides for the onset of
gastrulation and the realization of this action. It can be supposed that
after the formation of the same threshold amount of the products of
nuclear activity per unit of cytoplasmic volume, in eggs of different