CELL DIVISIONS
359
In animal embryos from different systematic groups the ratio of the
duration of different developmental periods is different : at the same
developmental stages they consist of cells of different cell generations
and have a different sum total duration of interkinetic states. The
differences of this kind can be found within one animal species if some
embryos are developing at average-optimal temperatures while others
at those close to the threshold ones.
A comparison of data on the latent differentiation of rudiments with
the sum total duration of interkinetic states of the cells at the same
developmental stages in embryos of various representatives of the
amphibians and sturgeon fishes, showed their changes to be correlated.
The same is found when comparing them in the embryos of the same
species developing under different temperatures or after a temporary
isolation of the material of the ecto- and entoderm in salt solution.
This suggests that the differences in the ratio of the duration of cleavage,
gastrulation and neurulation are one of the causes of differences in the
determination of similar rudiments at the same developmental stage.
A certain independence of the manifestation of the morphogenetic
function of nuclei is revealed which determines the onset of gastrulation
and neurulation and affects the competence and inductive properties of
the cells. The duration and the complex character of inductive influences
(in particular those in the development of sense organs) in amphibian
embryos, provide typical development consequent upon changes in the
ratios of individual developmental periods and, at the same time, limit
the range of this variability. In the case of the mosaic developmental
type such shifts encounter difficulties so that there exists a strict
correlation between the number of cell divisions, developmental stages
and differentiation.
In embryos of the same species the cells of different rudiments, in
particular those of the ectoderm and the entoderm, belong to different
cell generations and have a different relative sum total duration of interkinetic states of their cells. It can be thought that the asynchrony of
divisions is one of the ways in which the differentiation of the parts of
the embryo is realized.
In the subject of this survey evidence is often deficient and many of
the suggestions made must be regarded merely as working hypotheses.
One may cherish the hope, however, that precise data on mitotic indices
in different rudiments and at successive developmental stages, as well as
on the dimensionless characteristics of the duration of the periods of
cleavage, gastrulation and neurulation, along with the data on the onset
and periodicity of the morphogenetic function of nuclei and on the
degree of determination of rudiments at different developmental stages,
when once obtained will contribute to the understanding of the causes
359
In animal embryos from different systematic groups the ratio of the
duration of different developmental periods is different : at the same
developmental stages they consist of cells of different cell generations
and have a different sum total duration of interkinetic states. The
differences of this kind can be found within one animal species if some
embryos are developing at average-optimal temperatures while others
at those close to the threshold ones.
A comparison of data on the latent differentiation of rudiments with
the sum total duration of interkinetic states of the cells at the same
developmental stages in embryos of various representatives of the
amphibians and sturgeon fishes, showed their changes to be correlated.
The same is found when comparing them in the embryos of the same
species developing under different temperatures or after a temporary
isolation of the material of the ecto- and entoderm in salt solution.
This suggests that the differences in the ratio of the duration of cleavage,
gastrulation and neurulation are one of the causes of differences in the
determination of similar rudiments at the same developmental stage.
A certain independence of the manifestation of the morphogenetic
function of nuclei is revealed which determines the onset of gastrulation
and neurulation and affects the competence and inductive properties of
the cells. The duration and the complex character of inductive influences
(in particular those in the development of sense organs) in amphibian
embryos, provide typical development consequent upon changes in the
ratios of individual developmental periods and, at the same time, limit
the range of this variability. In the case of the mosaic developmental
type such shifts encounter difficulties so that there exists a strict
correlation between the number of cell divisions, developmental stages
and differentiation.
In embryos of the same species the cells of different rudiments, in
particular those of the ectoderm and the entoderm, belong to different
cell generations and have a different relative sum total duration of interkinetic states of their cells. It can be thought that the asynchrony of
divisions is one of the ways in which the differentiation of the parts of
the embryo is realized.
In the subject of this survey evidence is often deficient and many of
the suggestions made must be regarded merely as working hypotheses.
One may cherish the hope, however, that precise data on mitotic indices
in different rudiments and at successive developmental stages, as well as
on the dimensionless characteristics of the duration of the periods of
cleavage, gastrulation and neurulation, along with the data on the onset
and periodicity of the morphogenetic function of nuclei and on the
degree of determination of rudiments at different developmental stages,
when once obtained will contribute to the understanding of the causes