348
T. A. DETTLAFF
cleavage and gastrulation typical of optimal temperatures: at a low
temperature the tempo of cell divisions is higher than the rate of
gastrulation and, correspondingly, the level of latent differentiation and
0 2 consumption are higher. On the contrary, at a high temperature a
decrease in the rate of cell divisions with respect to the rate of gastrulation takes place ; correspondingly, latent differentiation is much weaker,
and 0 2 consumption is lower. It seems very likely that it is just this
change in the ratio of the developmental stages of the embryos and the
age of component cells, as well as the relative duration of the interkinetic state of cells, which are responsible for both the rate of latent
differentiation and the level of 0 2 consumption. The fact that in the
axolotl, with a smaller number of cells at the same developmental stages
as compared with the frog (IV, E), the curve of 0 2 consumption in early
developmental stages runs lower than that in the frog (Ten Cate, 1956)
favours the above assumption.
The correspondence described between the changes in the time of
latent differentiation and the changes in the ratio of the duration of
developmental periods exists, however, only within a certain temperature
range. The data of Jacobson (1955,1958) suggest that, when temperature
drops below a certain limit, the discrepancy between the age of cells
measured by τ 0 and developmental stage can turn out to be so great
that by the time of gastrulation and neurulation, i.e. at the moment of
normal interaction between the cells of the ectoderm and the mesoderm,
their reactivity is so much decreased (cf. IV, C) that their interaction
becomes disturbed. This seems to explain why in the experiments by
Jacobson (1958) at extremely low temperatures the percentage of
embryos with signs of early latent differentiation is smaller than at a
somewhat higher temperature, while the percentage of malformation
and death is higher.
At low temperatures which still preserve the normal reactivity of the
cells (i.e. when the values of TJ/TQ and τ π /τ 0 do not exceed a certain limit),
an increase in the relative duration of gastrulation is accompanied by an
increased relative duration of the contact between the ectoderm and
the mesoderm, i.e. a prolongation of the period of induction. The
problem of whether an earlier latent differentiation is, in these conditions,
a result of the longer contact, or a result of a higher competence of the
cells at the time of their interaction, or a result of both phenomena,
requires experimental investigation.
B. Effect of Li Ions
A correlation is found to exist between the processes of cell division
and differentiation in experiments with Li ions. A relative prolongation
of the period of synchronous division in the eggs of Paracentrotus
lividus
T. A. DETTLAFF
cleavage and gastrulation typical of optimal temperatures: at a low
temperature the tempo of cell divisions is higher than the rate of
gastrulation and, correspondingly, the level of latent differentiation and
0 2 consumption are higher. On the contrary, at a high temperature a
decrease in the rate of cell divisions with respect to the rate of gastrulation takes place ; correspondingly, latent differentiation is much weaker,
and 0 2 consumption is lower. It seems very likely that it is just this
change in the ratio of the developmental stages of the embryos and the
age of component cells, as well as the relative duration of the interkinetic state of cells, which are responsible for both the rate of latent
differentiation and the level of 0 2 consumption. The fact that in the
axolotl, with a smaller number of cells at the same developmental stages
as compared with the frog (IV, E), the curve of 0 2 consumption in early
developmental stages runs lower than that in the frog (Ten Cate, 1956)
favours the above assumption.
The correspondence described between the changes in the time of
latent differentiation and the changes in the ratio of the duration of
developmental periods exists, however, only within a certain temperature
range. The data of Jacobson (1955,1958) suggest that, when temperature
drops below a certain limit, the discrepancy between the age of cells
measured by τ 0 and developmental stage can turn out to be so great
that by the time of gastrulation and neurulation, i.e. at the moment of
normal interaction between the cells of the ectoderm and the mesoderm,
their reactivity is so much decreased (cf. IV, C) that their interaction
becomes disturbed. This seems to explain why in the experiments by
Jacobson (1958) at extremely low temperatures the percentage of
embryos with signs of early latent differentiation is smaller than at a
somewhat higher temperature, while the percentage of malformation
and death is higher.
At low temperatures which still preserve the normal reactivity of the
cells (i.e. when the values of TJ/TQ and τ π /τ 0 do not exceed a certain limit),
an increase in the relative duration of gastrulation is accompanied by an
increased relative duration of the contact between the ectoderm and
the mesoderm, i.e. a prolongation of the period of induction. The
problem of whether an earlier latent differentiation is, in these conditions,
a result of the longer contact, or a result of a higher competence of the
cells at the time of their interaction, or a result of both phenomena,
requires experimental investigation.
B. Effect of Li Ions
A correlation is found to exist between the processes of cell division
and differentiation in experiments with Li ions. A relative prolongation
of the period of synchronous division in the eggs of Paracentrotus
lividus