6. C E L L D I V I S I O N
215
3. Cytoplasmic Movements and Cytokinesis
Though less remarkable than the nuclear aspects and the behaviour
of the achromatic apparatus, cytoplasmic changes are no less essential
to division, nor are they confined to the last stages of mitosis.
The rounding up of cells, the isolation from their neighbours, the
modified movements of included particles all point towards changes in
permeability and surface tension of the cell membrane and variations
in viscosity of the cytoplasm. These properties are more difficult to
study in somatic animal cells, even in cultures, than in some plants or
eggs. But it may be dangerous to generalize from these. The problems of
cytokinesis have been reviewed recently by Roberts (1961).
In films of cell division, one of the most spectacular phenomena is
the superficial "bubbling" of the cytoplasm. It takes place usually in
the late anaphase and early telophase, but may often begin somewhat
earlier (at metaphase) and last longer (Hughes, 1952; Boss, 1955). The
blebs develop very rapidly, perhaps because of the escape of fluid cytoplasm through a local weakness of the cell membrane, then retract
more slowly. Each of them is visible for 15-40 sec (Hughes, 1952).
Boss (1955) suggests that they appear particularly in areas where the
R N A concentration is high. Sometimes the proximity of a chromosome
to the cell surface seems to favour a repeated local bubbling.
In cultured cells, cytokinesis seems less dependent on the position of
the spindle and the separation of chromosomes than in other materials
where the achromatic apparatus is bigger. The cytoplasm tends to
cleave in two whether the nuclear phenomena have reached their
normal conclusion or not. This has been observed in individual cases
after blocking of the spindle by beryllium or other substances
(Chevremont and Firket, 1952a). Under the influence of diethylstilboestrol and testosterone (Lettre and Siebs, 1956) or a-particle
irradiation of the spindle (Davis and Smith, 1957) sometimes chromosomes are not separated, but cytoplasmic cleavage is normal and gives
rise to a nucleated and an anucleated fragment of cytoplasm.
Cytokinesis is usually mechanically prevented when a single large
nucleus has reformed because of failure of anaphase (adenine; beryllium, etc.) but it can take place (for example after trypaflavine or
irradiation) when a chromosome bridge links the two groups. In this
case, the two daughter cells do not separate, but die together in
pycnosis.
Cytokinesis is dependent on an intact respiratory mechanism as
shown by its frequent failure after a normal or quasi-normal division,
when the cells are subjected to anaerobiosis (S. Chevremont and
Frederic, 1955) cyanide (Chevremont and Firket, 1950), or cysteamine
215
3. Cytoplasmic Movements and Cytokinesis
Though less remarkable than the nuclear aspects and the behaviour
of the achromatic apparatus, cytoplasmic changes are no less essential
to division, nor are they confined to the last stages of mitosis.
The rounding up of cells, the isolation from their neighbours, the
modified movements of included particles all point towards changes in
permeability and surface tension of the cell membrane and variations
in viscosity of the cytoplasm. These properties are more difficult to
study in somatic animal cells, even in cultures, than in some plants or
eggs. But it may be dangerous to generalize from these. The problems of
cytokinesis have been reviewed recently by Roberts (1961).
In films of cell division, one of the most spectacular phenomena is
the superficial "bubbling" of the cytoplasm. It takes place usually in
the late anaphase and early telophase, but may often begin somewhat
earlier (at metaphase) and last longer (Hughes, 1952; Boss, 1955). The
blebs develop very rapidly, perhaps because of the escape of fluid cytoplasm through a local weakness of the cell membrane, then retract
more slowly. Each of them is visible for 15-40 sec (Hughes, 1952).
Boss (1955) suggests that they appear particularly in areas where the
R N A concentration is high. Sometimes the proximity of a chromosome
to the cell surface seems to favour a repeated local bubbling.
In cultured cells, cytokinesis seems less dependent on the position of
the spindle and the separation of chromosomes than in other materials
where the achromatic apparatus is bigger. The cytoplasm tends to
cleave in two whether the nuclear phenomena have reached their
normal conclusion or not. This has been observed in individual cases
after blocking of the spindle by beryllium or other substances
(Chevremont and Firket, 1952a). Under the influence of diethylstilboestrol and testosterone (Lettre and Siebs, 1956) or a-particle
irradiation of the spindle (Davis and Smith, 1957) sometimes chromosomes are not separated, but cytoplasmic cleavage is normal and gives
rise to a nucleated and an anucleated fragment of cytoplasm.
Cytokinesis is usually mechanically prevented when a single large
nucleus has reformed because of failure of anaphase (adenine; beryllium, etc.) but it can take place (for example after trypaflavine or
irradiation) when a chromosome bridge links the two groups. In this
case, the two daughter cells do not separate, but die together in
pycnosis.
Cytokinesis is dependent on an intact respiratory mechanism as
shown by its frequent failure after a normal or quasi-normal division,
when the cells are subjected to anaerobiosis (S. Chevremont and
Frederic, 1955) cyanide (Chevremont and Firket, 1950), or cysteamine