CELL DIVISIONS
325
that stage (Dettlaff, 1956), and are also correlated with differences in the
relative duration of the same developmental periods, as shown below.
Thus the problem of the relationships between cleavage, relative
duration of developmental periods, and differentiation requires further
analysis.
I. Cleavage Divisions. Periods of Synchronous
and Asynchronous Division
Cleavage in the embryonic development of animals consists of two
qualitatively different periods : that of synchronous and that of asynchronous division.
The former is characterized, apart from the synchrony of nuclear
division and the constancy of the rhythm of cytoplasmic division, by the
karyomeric structure and minimal duration of the interphase state of
nuclei (Balinsky, 1931; Syngayewskaja, 1931; Agrell, 1958). With the
transition to the second period, the synchrony of nuclear division is
disturbed, first of all between the animal and vegetative blastomeres,
and then within both animal and vegetative regions. An increase in the
relative duration of interphase takes place at this stage, which is
manifested in a decrease in the mitotic index (M.I.) (Richards and
Porter, 1935; Richards and Schumacher, 1935; Bragg, 1938,1939; Jones,
1939).
According to Richards and Porter (1935) and Richards and
Schumacher (1935), the M.I. at the stages of synchronous division in
Fundulus heteroclitus and Goregonus clupeiformis is 90% or more ; resting
nuclei are almost absent in the early blastula, their number increasing
during the mid-blastula. At this time the M.I. is 83% in Coregonus, and
61% in Fundulus. After the onset of gastrulation the M.I. sharply falls
in both species to an average of 1-9 and 3-8% respectively. At the
blastula stage in Bufo cognathus Bragg (1938) found an M.I. of 60-9%,
falling during gastrulation from 55-4 to 7-7%.
According to the data of Schönmann (1938), the synchrony of divisions
becomes disturbed at the late blastula stage (this corresponds to Harrison
stage 7 and 8 for Ambystoma punctatum), the complete asynchrony at
which nuclei at all mitotic stages can be found in one section beginning
between stages 8 and 8 +. According to the data of Neyfakh and Rott
(1958), the synchrony of divisions in the loach (Misgurnus fossilis) and
sevruga (Acipenser stellatus) is disturbed at the mid-blastula stage.
Neyfakh and Rott found that the transition from synchronous to
asynchronous divisions proceeds very rapidly and seems to be independent of nuclear control, since X-ray-induced inactivation of nuclei does
not affect it. In sea urchins (Agrell, 1958) the asynchrony of divisions,
325
that stage (Dettlaff, 1956), and are also correlated with differences in the
relative duration of the same developmental periods, as shown below.
Thus the problem of the relationships between cleavage, relative
duration of developmental periods, and differentiation requires further
analysis.
I. Cleavage Divisions. Periods of Synchronous
and Asynchronous Division
Cleavage in the embryonic development of animals consists of two
qualitatively different periods : that of synchronous and that of asynchronous division.
The former is characterized, apart from the synchrony of nuclear
division and the constancy of the rhythm of cytoplasmic division, by the
karyomeric structure and minimal duration of the interphase state of
nuclei (Balinsky, 1931; Syngayewskaja, 1931; Agrell, 1958). With the
transition to the second period, the synchrony of nuclear division is
disturbed, first of all between the animal and vegetative blastomeres,
and then within both animal and vegetative regions. An increase in the
relative duration of interphase takes place at this stage, which is
manifested in a decrease in the mitotic index (M.I.) (Richards and
Porter, 1935; Richards and Schumacher, 1935; Bragg, 1938,1939; Jones,
1939).
According to Richards and Porter (1935) and Richards and
Schumacher (1935), the M.I. at the stages of synchronous division in
Fundulus heteroclitus and Goregonus clupeiformis is 90% or more ; resting
nuclei are almost absent in the early blastula, their number increasing
during the mid-blastula. At this time the M.I. is 83% in Coregonus, and
61% in Fundulus. After the onset of gastrulation the M.I. sharply falls
in both species to an average of 1-9 and 3-8% respectively. At the
blastula stage in Bufo cognathus Bragg (1938) found an M.I. of 60-9%,
falling during gastrulation from 55-4 to 7-7%.
According to the data of Schönmann (1938), the synchrony of divisions
becomes disturbed at the late blastula stage (this corresponds to Harrison
stage 7 and 8 for Ambystoma punctatum), the complete asynchrony at
which nuclei at all mitotic stages can be found in one section beginning
between stages 8 and 8 +. According to the data of Neyfakh and Rott
(1958), the synchrony of divisions in the loach (Misgurnus fossilis) and
sevruga (Acipenser stellatus) is disturbed at the mid-blastula stage.
Neyfakh and Rott found that the transition from synchronous to
asynchronous divisions proceeds very rapidly and seems to be independent of nuclear control, since X-ray-induced inactivation of nuclei does
not affect it. In sea urchins (Agrell, 1958) the asynchrony of divisions,