342
T. A. DETTLAFF
temperatures within the average range for each species. However, the
differences revealed are so great that it can hardly be thought that
further more precise data would reveal relationships different in principle.
For the cleavage period these data are in agreement with the results
of a recalculation of the number of nuclei in central sagittal sections
through the embryos of Rana terrestris, Ambystoma
mexicanum
and
Triturus vulgaris (Dettlaff, 1956). The sections represent relatively equal
portions of the embryo at the stage of the onset of gastrulation, and
contain the fewest cells in T. vulgaris, while the greatest number is
found in R. terrestris (Table II). The onset of gastrulation in the embryos
of the newt and axolotl occurs at earlier cleavage stages in comparison
with the frog embryos. At the same time, in newt embryos, a relatively
larger number of cells undergoes divisions in the periods of gastrulation
and neurulation than in the axolotl, while in the axolotl there are more
than that in the anurans. These observations require the final proof of a
direct count of mitotic coefficients, though their substantial correctness
is evident even upon a simple examination of the sections. This means
that not only are there a smaller number of cell divisions by the time of
onset of gastrulation but the sum total duration of the interkinetic state of
the nuclei during the period of gastrulation and neurulation is in the newt
smaller than in the axolotl, and smaller in the axolotl than in the frog.
It is of great significance (cf. V and VI) that the range of these
differences increases up to the onset of neurulation.
Unlike amphibians, in sturgeon species studied in this respect, the
relative duration of the same developmental periods is similar. τ η /τ 0 for
different periods in the sturgeon and sevruga have practically the same
values (Dettlaff and Dettlaff, 1960, 1961; Kroenig, 1960) (Table I).
Correspondingly (Kroenig, 1960) the number of nuclei in central
sagittal sections through the embryos at the very onset of gastrulation
in the sturgeon and sevruga is closely similar (Table II).
This implies that the transition to asynchronous divisions in the
sturgeon and sevruga takes place at the same cleavage stage, and
embryos at the same developmental stage consist of cells of the same
generation. In other words, the number and the sum total duration of
interkinetic states of nuclei over the whole development up to the same
stage in the sturgeon and sevruga are very close if not equal.
In white sturgeon embryos (Chulitskaya, 1961b) gastrulation begins
at somewhat later stages of cleavage (τ τ /τ 0 in white sturgeon is 22· 0-22· 3,
instead of 19-4-20-3 in sturgeon and sevruga) and, correspondingly, in
central sagittal sections through the early gastrula in the white sturgeon
a greater number of nuclei is present (Table II). At the beginning of
neurulation the differences in τ π /τ 0 between the sturgeon and sevruga, on
the one hand, and white sturgeon, on the other, amounts to 4-6 r 0 in
T. A. DETTLAFF
temperatures within the average range for each species. However, the
differences revealed are so great that it can hardly be thought that
further more precise data would reveal relationships different in principle.
For the cleavage period these data are in agreement with the results
of a recalculation of the number of nuclei in central sagittal sections
through the embryos of Rana terrestris, Ambystoma
mexicanum
and
Triturus vulgaris (Dettlaff, 1956). The sections represent relatively equal
portions of the embryo at the stage of the onset of gastrulation, and
contain the fewest cells in T. vulgaris, while the greatest number is
found in R. terrestris (Table II). The onset of gastrulation in the embryos
of the newt and axolotl occurs at earlier cleavage stages in comparison
with the frog embryos. At the same time, in newt embryos, a relatively
larger number of cells undergoes divisions in the periods of gastrulation
and neurulation than in the axolotl, while in the axolotl there are more
than that in the anurans. These observations require the final proof of a
direct count of mitotic coefficients, though their substantial correctness
is evident even upon a simple examination of the sections. This means
that not only are there a smaller number of cell divisions by the time of
onset of gastrulation but the sum total duration of the interkinetic state of
the nuclei during the period of gastrulation and neurulation is in the newt
smaller than in the axolotl, and smaller in the axolotl than in the frog.
It is of great significance (cf. V and VI) that the range of these
differences increases up to the onset of neurulation.
Unlike amphibians, in sturgeon species studied in this respect, the
relative duration of the same developmental periods is similar. τ η /τ 0 for
different periods in the sturgeon and sevruga have practically the same
values (Dettlaff and Dettlaff, 1960, 1961; Kroenig, 1960) (Table I).
Correspondingly (Kroenig, 1960) the number of nuclei in central
sagittal sections through the embryos at the very onset of gastrulation
in the sturgeon and sevruga is closely similar (Table II).
This implies that the transition to asynchronous divisions in the
sturgeon and sevruga takes place at the same cleavage stage, and
embryos at the same developmental stage consist of cells of the same
generation. In other words, the number and the sum total duration of
interkinetic states of nuclei over the whole development up to the same
stage in the sturgeon and sevruga are very close if not equal.
In white sturgeon embryos (Chulitskaya, 1961b) gastrulation begins
at somewhat later stages of cleavage (τ τ /τ 0 in white sturgeon is 22· 0-22· 3,
instead of 19-4-20-3 in sturgeon and sevruga) and, correspondingly, in
central sagittal sections through the early gastrula in the white sturgeon
a greater number of nuclei is present (Table II). At the beginning of
neurulation the differences in τ π /τ 0 between the sturgeon and sevruga, on
the one hand, and white sturgeon, on the other, amounts to 4-6 r 0 in