CELL DIVISIONS
353
tempo of latent differentiation of the ectoderm in the ear region of
Triton taeniatus, Ambystoma
mexicanum
and Rana terrestris. When
comparing them with the data on the relative duration of the periods of
cleavage, gastrulation and in part neurulation, and with the number of
nuclei in central sagittal sections at the onset of gastrulation in these
animals (cf. IV, Ε), it can easily be seen that the differences in the ratio
of cell divisions to formative processes correspond to differences in the
tempo of latent differentiation : the younger the cell generations at a given
developmental stage and the smaller the sum total duration of the
interkinetic state of nuclei from the onset of their functioning up to
this stage, the lower the level of latent differentiation of similar
rudiments.
In the embryos of sevruga and sturgeon the ear ectoderm was
explanted (Chulitskaya, 1961a, b) from successive developmental stages,
from the stage of a slit-shaped blastopore up to the stage of the appearance of eye outgrowths. Ear vesicles begin to form in expiants only from
the stage of the closing neural plate onwards, when the ear ectoderm
comes into contact with the rudiment of the hindbrain. At subsequent
stages the percentage of ear vesicles, including those with sensory areas,
rapidly increases. The dimensions of the vesicles increase concurrently.
The characteristics of the level of latent differentiation of the rudiment
of the ear vesicle in sevruga and sturgeon puzzlingly coincide (Fig. 5). In
the same conditions the ectoderm of the ear region in white sturgeon
forms complex vesicles starting from the same stage of the closing
neurula. At the same developmental stages, however, the percentage of
cases of vesicle formation in the white sturgeon is always higher than in
sevruga and sturgeon. The percentage of complex vesicles with sensory
areas is similar at the same stages in the white sturgeon, sturgeon and
sevruga, despite their relative smaller size in the white sturgeon as
compared to those in the sturgeon and sevruga.
Thus the similarity in the level of latent differentiation of the
presumptive ear vesicle corresponds in the sturgeon and sevruga to the
similarity in τ„/τ 0 relations and cell generation at the same developmental stages (IV, E). In white sturgeon embryos a small increase in
the number of cell generations is accompanied by a small increase in the
degree of determination of the material of the ear vesicle at the same
developmental stages. When comparing the regularities of latent
differentiation of the material of the ear vesicle in sturgeon fishes and
amphibians, it turns out that the underlying mesoderm of sturgeon
fishes plays an even smaller part in the process than it does in the
urodeles (Chulitskaya, 1962).
One of the causes of the differences mentioned in the rate of latent
differentiation, besides the differences in the ratios of cell divisions to
ο
353
tempo of latent differentiation of the ectoderm in the ear region of
Triton taeniatus, Ambystoma
mexicanum
and Rana terrestris. When
comparing them with the data on the relative duration of the periods of
cleavage, gastrulation and in part neurulation, and with the number of
nuclei in central sagittal sections at the onset of gastrulation in these
animals (cf. IV, Ε), it can easily be seen that the differences in the ratio
of cell divisions to formative processes correspond to differences in the
tempo of latent differentiation : the younger the cell generations at a given
developmental stage and the smaller the sum total duration of the
interkinetic state of nuclei from the onset of their functioning up to
this stage, the lower the level of latent differentiation of similar
rudiments.
In the embryos of sevruga and sturgeon the ear ectoderm was
explanted (Chulitskaya, 1961a, b) from successive developmental stages,
from the stage of a slit-shaped blastopore up to the stage of the appearance of eye outgrowths. Ear vesicles begin to form in expiants only from
the stage of the closing neural plate onwards, when the ear ectoderm
comes into contact with the rudiment of the hindbrain. At subsequent
stages the percentage of ear vesicles, including those with sensory areas,
rapidly increases. The dimensions of the vesicles increase concurrently.
The characteristics of the level of latent differentiation of the rudiment
of the ear vesicle in sevruga and sturgeon puzzlingly coincide (Fig. 5). In
the same conditions the ectoderm of the ear region in white sturgeon
forms complex vesicles starting from the same stage of the closing
neurula. At the same developmental stages, however, the percentage of
cases of vesicle formation in the white sturgeon is always higher than in
sevruga and sturgeon. The percentage of complex vesicles with sensory
areas is similar at the same stages in the white sturgeon, sturgeon and
sevruga, despite their relative smaller size in the white sturgeon as
compared to those in the sturgeon and sevruga.
Thus the similarity in the level of latent differentiation of the
presumptive ear vesicle corresponds in the sturgeon and sevruga to the
similarity in τ„/τ 0 relations and cell generation at the same developmental stages (IV, E). In white sturgeon embryos a small increase in
the number of cell generations is accompanied by a small increase in the
degree of determination of the material of the ear vesicle at the same
developmental stages. When comparing the regularities of latent
differentiation of the material of the ear vesicle in sturgeon fishes and
amphibians, it turns out that the underlying mesoderm of sturgeon
fishes plays an even smaller part in the process than it does in the
urodeles (Chulitskaya, 1962).
One of the causes of the differences mentioned in the rate of latent
differentiation, besides the differences in the ratios of cell divisions to
ο