228
H . F I R K E T
beginning of prophase (S. Chevremont et al. 9 1957; Firket, 1958a). The
only possible explanation is that some of the preparatory reactions
usually occuring during G 2 have continued at the lower temperature
and are not dependant on D N A synthesis. A similar "mixing" of the
S and G 2 phenomena after irradiation has been described (Painter,
1960).
In sea urchin eggs, as in many other cells, perhaps the very first
event of the cell cycle is the doubling of the centrioles during the
telophase of the previous cleavage. Thus, this doubling occurs long
before D N A synthesis. Bucher and Mazia (1960) inhibited this doubling
with mercaptoethanol, preventing the next division without affecting
D N A synthesis. Once again, two essential preparatory events are found
to be independent, though usually they follow each other in the same
sequence.
If the search for an unique trigger is vain, it does not mean we
cannot effectively favour or inhibit division. Unfortunately, although
long lists of favourable or unfavourable agents or circumstances can be
drawn up, the mechanisms of their action have not, in most cases, been
analysed sufficiently to identify the preparatory event upon which they
act. A few years ago, Swann (1957, 1958) published a remarkable
review of such effects. New active substances are discovered every year:
for example, oncotrephin (Ito, Matuda, Kosaki and Kuru, 1959),
phytohaemagglutinin (Nowell, 1960), etc. Phytohaemagglutinin enables human leucocytes to effect D N A synthesis in vitro (Bender and
Prescott, 1962). But rather than list these agents once again or try to
analyse their action, it is probably more useful to underline a few
pitfalls in their investigation and to raise some questions.
One of the main problems in the study of substances supposed to
favour division in cultures is the difficulty of separating this action from
purely nutritional or general "growth" effects (i.e. mass increase ofthe
cells). If growth and division cannot be separated in culture, it does not
mean they are always linked. Many cleaving eggs, which have a very
short cell cycle, do not increase their volume, their total protein or
their R N A content, in fact, do not "grow" until the morula stage (see
Brachet, 1950, for references). It may also be worth mentioning that
several Protozoa may have part of the cytoplasm removed daily with
only a slight reduction in their rate of division (Hirschfield, Tulchin
and Fong, 1960).
Inhibiting substances, on the other hand, are only of interest if they
act specifically on some process of division. Adverse nutritional conditions
or poisons of metabolism in general will prevent a cell from dividing
because it has to remain reasonably healthy to be able to do so. But this
kind of effect will teach us nothing about the actual mechanism of
H . F I R K E T
beginning of prophase (S. Chevremont et al. 9 1957; Firket, 1958a). The
only possible explanation is that some of the preparatory reactions
usually occuring during G 2 have continued at the lower temperature
and are not dependant on D N A synthesis. A similar "mixing" of the
S and G 2 phenomena after irradiation has been described (Painter,
1960).
In sea urchin eggs, as in many other cells, perhaps the very first
event of the cell cycle is the doubling of the centrioles during the
telophase of the previous cleavage. Thus, this doubling occurs long
before D N A synthesis. Bucher and Mazia (1960) inhibited this doubling
with mercaptoethanol, preventing the next division without affecting
D N A synthesis. Once again, two essential preparatory events are found
to be independent, though usually they follow each other in the same
sequence.
If the search for an unique trigger is vain, it does not mean we
cannot effectively favour or inhibit division. Unfortunately, although
long lists of favourable or unfavourable agents or circumstances can be
drawn up, the mechanisms of their action have not, in most cases, been
analysed sufficiently to identify the preparatory event upon which they
act. A few years ago, Swann (1957, 1958) published a remarkable
review of such effects. New active substances are discovered every year:
for example, oncotrephin (Ito, Matuda, Kosaki and Kuru, 1959),
phytohaemagglutinin (Nowell, 1960), etc. Phytohaemagglutinin enables human leucocytes to effect D N A synthesis in vitro (Bender and
Prescott, 1962). But rather than list these agents once again or try to
analyse their action, it is probably more useful to underline a few
pitfalls in their investigation and to raise some questions.
One of the main problems in the study of substances supposed to
favour division in cultures is the difficulty of separating this action from
purely nutritional or general "growth" effects (i.e. mass increase ofthe
cells). If growth and division cannot be separated in culture, it does not
mean they are always linked. Many cleaving eggs, which have a very
short cell cycle, do not increase their volume, their total protein or
their R N A content, in fact, do not "grow" until the morula stage (see
Brachet, 1950, for references). It may also be worth mentioning that
several Protozoa may have part of the cytoplasm removed daily with
only a slight reduction in their rate of division (Hirschfield, Tulchin
and Fong, 1960).
Inhibiting substances, on the other hand, are only of interest if they
act specifically on some process of division. Adverse nutritional conditions
or poisons of metabolism in general will prevent a cell from dividing
because it has to remain reasonably healthy to be able to do so. But this
kind of effect will teach us nothing about the actual mechanism of