218
H . F I R K E T
anaerobiosis, trihydroxy-jV-methyl-indole, adrenochrome, acid deoxyribonuclease; see reviews in Frederic, 1958 and M . Chevremont, 1961),
entry into mitosis is prevented. A preprophasic block occurs. Curiously,
some of these substances (deoxyribonuclease, trihydroxy-JV-methylindole) cause the appearance of DNA-like material (Feulgen-positive
and incorporating tritiated thymidine) in the mitochondria (Chevremont, Chevremont and Baeckeland, 1959; Chevremont and Baeckeland, 1960). This substance may later disappear when the cells are
returned to normal conditions.
In spite of its importance before division, the chondriome seems to
have a more passive role during mitosis. In fibroblasts, movements of
mitochondria are slowed down and stop during prometaphase. Mitochondria are broken into shorter fragments, become thinner, and some
of them lose their contrast so that they become indistinguishable under
the phase microscope. They do not split longitudinally as was formerly
supposed but are distributed passively at telophase among the daughter
cells, according to the chance of their place in the cytoplasm. Soon,
their contrast increases again and their movements become more
active ( M . Chevremont and Frederic, 1952). Mitochondrial and
nucleolar changes during division are thus curiously parallel. The
mitochondria never disappear completely, however, and no change of
structure is observed in them with the electron microscope throughout
division (Bernhard, personal communication). The lowering of contrast
may find two explanations which are not mutually exclusive. An
increase in the refringence of the cytoplasm which is often observed
would reduce the difference between the indices of the cytoplasm and
the mitochondria. This could account for their temporary loss of
contrast. Or the mitochondria lose molecules that do not constitute an
essential part of their structure. The same substances or similar ones
would be reabsorbed rapidly at the end of division as shown by the
rapid rise in contrast at telophase.
I I I .
T H E C E L L C Y C L E
As our knowledge of the physiology of cell division increases, many
of its aspects are found to be the results of phenomena occurring before
the onset of mitosis, at various moments during the period which, in
the past, has been so badly named "the resting stage". A study of the
cell cycle, defined as the sequence of events affecting the cell emerging
from a division until the end of the following mitosis, is now essential.
A. D U R A T I O N OF T H E C Y C L E
Evaluation of the duration of the cell cycle (generation time), based
H . F I R K E T
anaerobiosis, trihydroxy-jV-methyl-indole, adrenochrome, acid deoxyribonuclease; see reviews in Frederic, 1958 and M . Chevremont, 1961),
entry into mitosis is prevented. A preprophasic block occurs. Curiously,
some of these substances (deoxyribonuclease, trihydroxy-JV-methylindole) cause the appearance of DNA-like material (Feulgen-positive
and incorporating tritiated thymidine) in the mitochondria (Chevremont, Chevremont and Baeckeland, 1959; Chevremont and Baeckeland, 1960). This substance may later disappear when the cells are
returned to normal conditions.
In spite of its importance before division, the chondriome seems to
have a more passive role during mitosis. In fibroblasts, movements of
mitochondria are slowed down and stop during prometaphase. Mitochondria are broken into shorter fragments, become thinner, and some
of them lose their contrast so that they become indistinguishable under
the phase microscope. They do not split longitudinally as was formerly
supposed but are distributed passively at telophase among the daughter
cells, according to the chance of their place in the cytoplasm. Soon,
their contrast increases again and their movements become more
active ( M . Chevremont and Frederic, 1952). Mitochondrial and
nucleolar changes during division are thus curiously parallel. The
mitochondria never disappear completely, however, and no change of
structure is observed in them with the electron microscope throughout
division (Bernhard, personal communication). The lowering of contrast
may find two explanations which are not mutually exclusive. An
increase in the refringence of the cytoplasm which is often observed
would reduce the difference between the indices of the cytoplasm and
the mitochondria. This could account for their temporary loss of
contrast. Or the mitochondria lose molecules that do not constitute an
essential part of their structure. The same substances or similar ones
would be reabsorbed rapidly at the end of division as shown by the
rapid rise in contrast at telophase.
I I I .
T H E C E L L C Y C L E
As our knowledge of the physiology of cell division increases, many
of its aspects are found to be the results of phenomena occurring before
the onset of mitosis, at various moments during the period which, in
the past, has been so badly named "the resting stage". A study of the
cell cycle, defined as the sequence of events affecting the cell emerging
from a division until the end of the following mitosis, is now essential.
A. D U R A T I O N OF T H E C Y C L E
Evaluation of the duration of the cell cycle (generation time), based