CELL DIVISIONS
355
late blastula (st. 9, Shumway, 1940), they function uninterruptedly
during the whole period of gastrulation and neurulation. Thus according
to the data in Neyfakh's paper (1961, Fig. 4) subsequently confirmed by
him, the activity of the nuclei in the anura from the onset of nuclear
function up to the early gastrula stage (up to the moment when nuclear
function in sturgeon fish eggs is interrupted) ensures the development of
the embryo up to the middle of neurulation : almost up to the same
stage as in sturgeon fishes. It is natural that, if in sturgeon fishes an
interruption takes place in morphogenetic nuclear functioning from the
middle of gastrulation up to the end of neurulation, while in anurans
the nuclei function throughout, latent differentiation in the latter at the
same developmental stages must have proceeded further.
Unfortunately there are no data available on radiation damage of
nuclei in the urodeles, so that we cannot judge whether the differences
in the ratios of cell generations and in the duration of the interkinetic
state of cells in the anura and urodeles at the same developmental
stages are coupled with differences in periodicity of nuclear function.
The examples presented show that the morphogenetic function of
nuclei during the period of gastrulation and neurulation, as well as
during that of asynchronous divisions at the blastula stage (III, A)
affects latent differentiation (the competence of the rudiments or the
manifestation of inducing effects, or both). On the other hand, these
examples show that the differences in the ratio of cell divisions by the
onset of gastrulation, and the differences in the ratio of the dimensionless duration of gastrulation and neurulation in related animal groups,
are of morphological significance.
Examples of a relationship between the number of cell divisions, the
duration of the interkinetic state of nuclei and the tempo of latent
differentiation would be useless if they did not hold true for other
organogeneses as well. In this connection it is necessary to touch upon
the problem of the ratio of the processes of latent differentiation of the
derivatives of different germinal layers. The regular character of these
ratios is most clearly revealed when they are compared in different
animals.
VI. The Ratio of the Rate of Processes of Latent Differentiation
in the Derivatives of Different Germ Layers in Amphibian Embryos
The very first work of Spemann (1912) showed that the ability of the
lens-forming epithelium to form free lenses increased in parallel with
the loss of this ability in the remaining epithelium when an eye rudiment
was transplanted under it. At the same time Spemann and many other
investigators after him (cf. Twitty, 1955; Lopashov and Stroeva, 1961)
355
late blastula (st. 9, Shumway, 1940), they function uninterruptedly
during the whole period of gastrulation and neurulation. Thus according
to the data in Neyfakh's paper (1961, Fig. 4) subsequently confirmed by
him, the activity of the nuclei in the anura from the onset of nuclear
function up to the early gastrula stage (up to the moment when nuclear
function in sturgeon fish eggs is interrupted) ensures the development of
the embryo up to the middle of neurulation : almost up to the same
stage as in sturgeon fishes. It is natural that, if in sturgeon fishes an
interruption takes place in morphogenetic nuclear functioning from the
middle of gastrulation up to the end of neurulation, while in anurans
the nuclei function throughout, latent differentiation in the latter at the
same developmental stages must have proceeded further.
Unfortunately there are no data available on radiation damage of
nuclei in the urodeles, so that we cannot judge whether the differences
in the ratios of cell generations and in the duration of the interkinetic
state of cells in the anura and urodeles at the same developmental
stages are coupled with differences in periodicity of nuclear function.
The examples presented show that the morphogenetic function of
nuclei during the period of gastrulation and neurulation, as well as
during that of asynchronous divisions at the blastula stage (III, A)
affects latent differentiation (the competence of the rudiments or the
manifestation of inducing effects, or both). On the other hand, these
examples show that the differences in the ratio of cell divisions by the
onset of gastrulation, and the differences in the ratio of the dimensionless duration of gastrulation and neurulation in related animal groups,
are of morphological significance.
Examples of a relationship between the number of cell divisions, the
duration of the interkinetic state of nuclei and the tempo of latent
differentiation would be useless if they did not hold true for other
organogeneses as well. In this connection it is necessary to touch upon
the problem of the ratio of the processes of latent differentiation of the
derivatives of different germinal layers. The regular character of these
ratios is most clearly revealed when they are compared in different
animals.
VI. The Ratio of the Rate of Processes of Latent Differentiation
in the Derivatives of Different Germ Layers in Amphibian Embryos
The very first work of Spemann (1912) showed that the ability of the
lens-forming epithelium to form free lenses increased in parallel with
the loss of this ability in the remaining epithelium when an eye rudiment
was transplanted under it. At the same time Spemann and many other
investigators after him (cf. Twitty, 1955; Lopashov and Stroeva, 1961)