332
T. A. DETTLAFF
However, there are few data available in the literature for the amphibians
and fishes (e.g. Sze, 1953, for R. pipiens). An attempt in our laboratory
to use the dissociation of the embryo in EDTA solution for this purpose
has not so far given results, since great difficulties have been encountered
in the counting of a large number of cells of different sizes. Cell numbers
could be determined by the procedures for nuclear isolation used in
biochemical investigations. These methods, however, do not ensure a
quantitative isolation of nuclei, or at any rate they have not been
validated from this point of view. Determination of the number of cells
by the amount of DNA was also not suitable since one cannot be quite
sure that the cytoplasmic DNA has been completely exhausted at the
onset of gastrulation (Sze, 1953).
Therefore, in order to judge the degree of constancy of the number of
cleavage divisions at the onset of gastrulation, data on the relative
duration of the cleavage period were used. The duration of one cleavage
division (τ 0 ) in the period of synchronous division at a constant
temperature was suggested as a unit characteristic of the relative
duration of embryonic development (Dettlaff and Dettlaff, 1960,
1961). The period between the appearance of the furrows of the first
and of the second cleavage division on the egg surface is used. If the
duration of the cleavage period, from insemination to the onset of
gastrulation (τ τ ) is divided by the duration of one cleavage division τ 0 ,
a dimensionless characteristic of the duration of the cleavage τ τ /τ 0 is
obtained which is comparable at different temperatures (Dettlaff and
Dettlaff, 1961). ti/t 0 does not correspond to the actual number of cleavage
divisions which an egg undergoes up to the onset of gastrulation, since
during asynchronous division their tempo somewhat decreases. However, since the temperature dépendance of the rhythm of divisions
during the periods of synchronous and asynchronous cleavage is the
same (cf. Dettlaff, 1963) the ratio of tj/tq at two temperatures t x and t 2
within the optimal range equals the ratio of the number of cell divisions
during the period r x at these temperatures.
A study of TJ/TQ in sturgeon (Acipenser güldenstädti) and sevruga
(A. stellatus) showed that within the range of average spawning
temperature the value of TJ/TO was rather constant, averaging 203 ±0-6
(Dettlaff and Dettlaff, 1960, 1961). The same values were also obtained
by Chulitskaya (Kroenig, 1960). The value in white sturgeon (Huso huso ;
Chulitskaya, 1961b) is also constant at different temperatures, but is
larger than that in the sturgeon and sevruga, fluctuating around 22.
The data obtained are supported by the results of counting the number
of nuclei in embryos which have been developing at different temperatures. The number of nuclei was counted in central sagittal sections
through sturgeon and sevruga embryos at the onset of gastrulation.
T. A. DETTLAFF
However, there are few data available in the literature for the amphibians
and fishes (e.g. Sze, 1953, for R. pipiens). An attempt in our laboratory
to use the dissociation of the embryo in EDTA solution for this purpose
has not so far given results, since great difficulties have been encountered
in the counting of a large number of cells of different sizes. Cell numbers
could be determined by the procedures for nuclear isolation used in
biochemical investigations. These methods, however, do not ensure a
quantitative isolation of nuclei, or at any rate they have not been
validated from this point of view. Determination of the number of cells
by the amount of DNA was also not suitable since one cannot be quite
sure that the cytoplasmic DNA has been completely exhausted at the
onset of gastrulation (Sze, 1953).
Therefore, in order to judge the degree of constancy of the number of
cleavage divisions at the onset of gastrulation, data on the relative
duration of the cleavage period were used. The duration of one cleavage
division (τ 0 ) in the period of synchronous division at a constant
temperature was suggested as a unit characteristic of the relative
duration of embryonic development (Dettlaff and Dettlaff, 1960,
1961). The period between the appearance of the furrows of the first
and of the second cleavage division on the egg surface is used. If the
duration of the cleavage period, from insemination to the onset of
gastrulation (τ τ ) is divided by the duration of one cleavage division τ 0 ,
a dimensionless characteristic of the duration of the cleavage τ τ /τ 0 is
obtained which is comparable at different temperatures (Dettlaff and
Dettlaff, 1961). ti/t 0 does not correspond to the actual number of cleavage
divisions which an egg undergoes up to the onset of gastrulation, since
during asynchronous division their tempo somewhat decreases. However, since the temperature dépendance of the rhythm of divisions
during the periods of synchronous and asynchronous cleavage is the
same (cf. Dettlaff, 1963) the ratio of tj/tq at two temperatures t x and t 2
within the optimal range equals the ratio of the number of cell divisions
during the period r x at these temperatures.
A study of TJ/TQ in sturgeon (Acipenser güldenstädti) and sevruga
(A. stellatus) showed that within the range of average spawning
temperature the value of TJ/TO was rather constant, averaging 203 ±0-6
(Dettlaff and Dettlaff, 1960, 1961). The same values were also obtained
by Chulitskaya (Kroenig, 1960). The value in white sturgeon (Huso huso ;
Chulitskaya, 1961b) is also constant at different temperatures, but is
larger than that in the sturgeon and sevruga, fluctuating around 22.
The data obtained are supported by the results of counting the number
of nuclei in embryos which have been developing at different temperatures. The number of nuclei was counted in central sagittal sections
through sturgeon and sevruga embryos at the onset of gastrulation.