6 . C E L L D I V I S I O N
211
divisions. So in this case the substances of the spindle have not to be
synthesized anew before each division.
The possibility to study the spindle in cells where it does not reach
such giant proportions should not be underestimated and considerable
information has come from vertebrate cells in cultures. There is no
longer any controversy about the existence of fibres in the achromatic apparatus of the living cells. The spindle is not visible in phase
contrast, but the polarizing microscope leaves no doubt about its
orientated fibrillar structure. It is more developed in amphibian cells
(Hughes and Swann, 1948; Boss, 1954b; E. W . Taylor, 1959) but it has
also been analysed in human cells in culture (E. W . Taylor, 1960). In
all these cases, the centrioles separate during prophase with a spindle
between them on the side of the nucleus. After this first elongation of
the spindle, there is a temporary shortening during prometaphase
(E. W . Taylor, 1959, 1960).
The essential anaphase movements occur in two successive stages;
the chromosomes separate and go up to near the poles. Then the length
of the cell increases together with that of the spindle, while the cell
tends to become narrower (Fig. 5). This second movement, preparing
for the telophase pinching of the central zone of the cytoplasm, brings
the chromosomes much further apart. The two movements proceed at
different rates and are quite separable in the newt (Boss, 1954b), less
so in the chick (Hughes and Swann, 1948). Birefringence disappears
first between the two chromosomal groups where R N A is being shed
by them. The importance of these observations is that they demonstrate
two different mechanisms in the process of chromosome separation, for
which explanations must be found. Substances like adenine (Hughes,
1950, 1952) do not impair the first movement but inhibit the second,
which is probably more related to cytoplasmic division.
The R N A released into the spindle may have a function in anaphase
movement. Without relying too much on Mota's hypothesis (1956) of a
jet-engine-like progress of the chromosomes, metaphase block is o b -
served in cultures after addition of ribonuclease (Montgomery and
Bonner, 1959). Similar effects had been observed in plants (Kaufmann
and Das, 1955) and embryos (Brachet and Ledoux, 1955).
Few cytological phenomena have been submitted to such varied
pharmacological attacks * as the achromatic apparatus of the dividing
cell. The concentration at which many of these substances act on the
spindle is not much lower than that at which they impair cell life. It is
thus doubtful whether they have much specificity. This criticism does
*We cannot centre this chapter on the problem of mitotic poisons whether acting on the
spindle or not. Recent reviews of different aspects of these are found in Dustin (1956),
Biesele (1958, 1961,1962), Chevremont (1961), Siebs (1960) andTurchini and Sentein (I960).
211
divisions. So in this case the substances of the spindle have not to be
synthesized anew before each division.
The possibility to study the spindle in cells where it does not reach
such giant proportions should not be underestimated and considerable
information has come from vertebrate cells in cultures. There is no
longer any controversy about the existence of fibres in the achromatic apparatus of the living cells. The spindle is not visible in phase
contrast, but the polarizing microscope leaves no doubt about its
orientated fibrillar structure. It is more developed in amphibian cells
(Hughes and Swann, 1948; Boss, 1954b; E. W . Taylor, 1959) but it has
also been analysed in human cells in culture (E. W . Taylor, 1960). In
all these cases, the centrioles separate during prophase with a spindle
between them on the side of the nucleus. After this first elongation of
the spindle, there is a temporary shortening during prometaphase
(E. W . Taylor, 1959, 1960).
The essential anaphase movements occur in two successive stages;
the chromosomes separate and go up to near the poles. Then the length
of the cell increases together with that of the spindle, while the cell
tends to become narrower (Fig. 5). This second movement, preparing
for the telophase pinching of the central zone of the cytoplasm, brings
the chromosomes much further apart. The two movements proceed at
different rates and are quite separable in the newt (Boss, 1954b), less
so in the chick (Hughes and Swann, 1948). Birefringence disappears
first between the two chromosomal groups where R N A is being shed
by them. The importance of these observations is that they demonstrate
two different mechanisms in the process of chromosome separation, for
which explanations must be found. Substances like adenine (Hughes,
1950, 1952) do not impair the first movement but inhibit the second,
which is probably more related to cytoplasmic division.
The R N A released into the spindle may have a function in anaphase
movement. Without relying too much on Mota's hypothesis (1956) of a
jet-engine-like progress of the chromosomes, metaphase block is o b -
served in cultures after addition of ribonuclease (Montgomery and
Bonner, 1959). Similar effects had been observed in plants (Kaufmann
and Das, 1955) and embryos (Brachet and Ledoux, 1955).
Few cytological phenomena have been submitted to such varied
pharmacological attacks * as the achromatic apparatus of the dividing
cell. The concentration at which many of these substances act on the
spindle is not much lower than that at which they impair cell life. It is
thus doubtful whether they have much specificity. This criticism does
*We cannot centre this chapter on the problem of mitotic poisons whether acting on the
spindle or not. Recent reviews of different aspects of these are found in Dustin (1956),
Biesele (1958, 1961,1962), Chevremont (1961), Siebs (1960) andTurchini and Sentein (I960).