336
T. A. DETTLAFF
in ploidy bring about a compensatory change in cell number is a shift of
the time of onset of gastrulation in relation to the cleavage stages. The
regulation of the sizes of nuclei and cells seems to represent a more
complicated phenomenon, of which the shift of gastrulation onset is
only a component.
A strict correspondence between the number of cells at the same
developmental stage and the degree of ploidy in a given species may
serve as direct evidence of a correlation between the number of cell
divisions and the onset of gastrulation which is regulated by the activity
of the nuclei, in particular by the degree of their ploidy.
C. Relative Duration of the Periods of Gastrulation and Neurulation
Asynchronous cell divisions, starting from the blastula stage,
proceed during gastrulation and neurulation. The sum total duration of
the interphase state of nuclei during these periods is directly proportional
to the duration of gastrulation and neurulation and inversely proportional (corrected for duration of mitosis) to the mitotic index. The
mitotic index diminishes after the onset of asynchrony, and drops
sharply being measured only in units, by the onset of gastrulation or during its first stages (cf. Bragg, 1938, 1939; Jones, 1939; Richards and
Porter, 1935; Richards and Schumacher, 1935; Briggs, 1947; Agrell
1956 ; Neyfakh and Rott, 1958). As to the duration of the periods of gastrulation and neurulation, it was shown in sturgeon species (Dettlaff and
Dettlaff, 1960, 1961; Chulitskaya, 1961a, b) in Ambystoma
mexicanum
(Ten Cate, 1956; Skoblina, 1963) and in various anuran species (data of
J. Moore, 1939, cf. Dettlaff and Dettlaff, 1961, Skoblina, below) that, as
with the period of cleavage, within the zone of optimal temperatures
they are characterized in each animal species by their own, relatively
constant, r n jr 0 (Figs. 2 and 3). The same holds true for later periods of
embryonic development. Since the mitotic index is not high after the
onset of gastrulation, the majority of the cells is found in the interkinetic
state during gastrulation and neurulation. Thus the constancy of the
relative duration of these periods implies that each developmental stage
under normal conditions is characterized by a certain sum total
duration of the interkinetic state of the cells and by a certain, though
small, number of cell divisions.
It should be added that the correlation between the number of cell
divisions and the developmental stage seems also to exist in sea urchins.
This is supported by the fact that at eclosion sea urchin embryos
contain a definite number of cells: 500-526 cells in
Sphaerechinus
granulans
(Morgan, 1895a) 100 in Echinus microtuberculatus
(Morgan,
1895b), about 1500, in Echinus esculentus (Agrell and Persson, 1956),
about 500-600 in Arbacia (Harvey, 1940).
T. A. DETTLAFF
in ploidy bring about a compensatory change in cell number is a shift of
the time of onset of gastrulation in relation to the cleavage stages. The
regulation of the sizes of nuclei and cells seems to represent a more
complicated phenomenon, of which the shift of gastrulation onset is
only a component.
A strict correspondence between the number of cells at the same
developmental stage and the degree of ploidy in a given species may
serve as direct evidence of a correlation between the number of cell
divisions and the onset of gastrulation which is regulated by the activity
of the nuclei, in particular by the degree of their ploidy.
C. Relative Duration of the Periods of Gastrulation and Neurulation
Asynchronous cell divisions, starting from the blastula stage,
proceed during gastrulation and neurulation. The sum total duration of
the interphase state of nuclei during these periods is directly proportional
to the duration of gastrulation and neurulation and inversely proportional (corrected for duration of mitosis) to the mitotic index. The
mitotic index diminishes after the onset of asynchrony, and drops
sharply being measured only in units, by the onset of gastrulation or during its first stages (cf. Bragg, 1938, 1939; Jones, 1939; Richards and
Porter, 1935; Richards and Schumacher, 1935; Briggs, 1947; Agrell
1956 ; Neyfakh and Rott, 1958). As to the duration of the periods of gastrulation and neurulation, it was shown in sturgeon species (Dettlaff and
Dettlaff, 1960, 1961; Chulitskaya, 1961a, b) in Ambystoma
mexicanum
(Ten Cate, 1956; Skoblina, 1963) and in various anuran species (data of
J. Moore, 1939, cf. Dettlaff and Dettlaff, 1961, Skoblina, below) that, as
with the period of cleavage, within the zone of optimal temperatures
they are characterized in each animal species by their own, relatively
constant, r n jr 0 (Figs. 2 and 3). The same holds true for later periods of
embryonic development. Since the mitotic index is not high after the
onset of gastrulation, the majority of the cells is found in the interkinetic
state during gastrulation and neurulation. Thus the constancy of the
relative duration of these periods implies that each developmental stage
under normal conditions is characterized by a certain sum total
duration of the interkinetic state of the cells and by a certain, though
small, number of cell divisions.
It should be added that the correlation between the number of cell
divisions and the developmental stage seems also to exist in sea urchins.
This is supported by the fact that at eclosion sea urchin embryos
contain a definite number of cells: 500-526 cells in
Sphaerechinus
granulans
(Morgan, 1895a) 100 in Echinus microtuberculatus
(Morgan,
1895b), about 1500, in Echinus esculentus (Agrell and Persson, 1956),
about 500-600 in Arbacia (Harvey, 1940).