CELL DIVISIONS
333
Sections of equal thickness of sturgeon embryos which had developed at
12° and 18° C and of sevruga embryos developed at 13-8° and 18-19° C
contain practically the same number of nuclei (Chulitskaya, unpublished). This bears witness that gastrulation in sturgeon species starts at
a particular cleavage period, or in other words that the number of
cleavage divisions is related to the onset of gastrulation. Similar data
were obtained for embryos of Ambystoma mexicanum at 18° and 22° C
(Skoblina, 1964). At these temperatures the T T */T 0 ratio (where r T * is the
period from the appearance of the first cleavage furrow to the onset of
gastrulation) in advanced eggs varies from 15-3 to 16-0. The interval
from the moment of insemination to the appearance of the first cleavage
furrow is approximately 3r 0 ; a more precise determination of this
period is difficult because spawning and fertilization do not always
coincide in axolotls.
The consistency of the relative duration of cleavage at different
temperatures in Ambystoma mexicanum was observed by Ten Cate
(1956); he expressed its duration in fractions of the period from
fertilization to the end of neurulation (till Harrison stage 20). Applying
the curve of r 0 as a function of temperature plotted for Ambystoma
mexicanum (Skoblina, 1963) τ 0 values were obtained for the temperatures at which Ten Cate's embryos had developed. The value of τ 0 at
16° and 26° C according to the data of Ten Cate and Skoblina are very
close. At other temperatures the coincidence between their data is less
complete, which may be explained by the fact that Ten Cate registered
the time of onset of successive stages with a reliability of only ± 11 % of
their duration.
The relative duration of the cleavage period at different temperatures
(within the optimal range) also tends to be constant in American species
oîRana (R. sylvatica, R. pipiens, R. palustris), as shown by recalculation
of J. Moore's (1939) data (cf. Dettlaff and Dettlaff, 1961).
Thus the conclusion that a correlation exists between the duration of
the cleavage period, the number of cleavage divisions and the onset of
gastrulation in each animal species acquires a broader significance. This
conclusion is also supported by the development of haploid and polyploid
amphibian embryos.
B. Relationship of Cleavage Processes and Gastrulation in
Haploid and Polyploid Embryos
It is known (cf. Porter, 1939; Fankhauser, 1945, 1952; Briggs, 1947
and others) that in haploid and polyploid amphibian embryos, the ratio
normal for the given degree of ploidy between the size of nucleus and
cytoplasm, and between the size and number of cells in the embryo, are
established by the onset of gastrulation or during its first stages (e.g.
333
Sections of equal thickness of sturgeon embryos which had developed at
12° and 18° C and of sevruga embryos developed at 13-8° and 18-19° C
contain practically the same number of nuclei (Chulitskaya, unpublished). This bears witness that gastrulation in sturgeon species starts at
a particular cleavage period, or in other words that the number of
cleavage divisions is related to the onset of gastrulation. Similar data
were obtained for embryos of Ambystoma mexicanum at 18° and 22° C
(Skoblina, 1964). At these temperatures the T T */T 0 ratio (where r T * is the
period from the appearance of the first cleavage furrow to the onset of
gastrulation) in advanced eggs varies from 15-3 to 16-0. The interval
from the moment of insemination to the appearance of the first cleavage
furrow is approximately 3r 0 ; a more precise determination of this
period is difficult because spawning and fertilization do not always
coincide in axolotls.
The consistency of the relative duration of cleavage at different
temperatures in Ambystoma mexicanum was observed by Ten Cate
(1956); he expressed its duration in fractions of the period from
fertilization to the end of neurulation (till Harrison stage 20). Applying
the curve of r 0 as a function of temperature plotted for Ambystoma
mexicanum (Skoblina, 1963) τ 0 values were obtained for the temperatures at which Ten Cate's embryos had developed. The value of τ 0 at
16° and 26° C according to the data of Ten Cate and Skoblina are very
close. At other temperatures the coincidence between their data is less
complete, which may be explained by the fact that Ten Cate registered
the time of onset of successive stages with a reliability of only ± 11 % of
their duration.
The relative duration of the cleavage period at different temperatures
(within the optimal range) also tends to be constant in American species
oîRana (R. sylvatica, R. pipiens, R. palustris), as shown by recalculation
of J. Moore's (1939) data (cf. Dettlaff and Dettlaff, 1961).
Thus the conclusion that a correlation exists between the duration of
the cleavage period, the number of cleavage divisions and the onset of
gastrulation in each animal species acquires a broader significance. This
conclusion is also supported by the development of haploid and polyploid
amphibian embryos.
B. Relationship of Cleavage Processes and Gastrulation in
Haploid and Polyploid Embryos
It is known (cf. Porter, 1939; Fankhauser, 1945, 1952; Briggs, 1947
and others) that in haploid and polyploid amphibian embryos, the ratio
normal for the given degree of ploidy between the size of nucleus and
cytoplasm, and between the size and number of cells in the embryo, are
established by the onset of gastrulation or during its first stages (e.g.