MORPHOGENESIS IN
MICRASTERIAS
55
radiate form of this species about twenty in every thousand divisions
resulted in the formation of the normal biradiate form. This shows that
the aradiate form is at all events much more stable than the uniradiate
one. The only sign of a lateral differentiation so far seen was represented by a single cell among thousands which had a spine-like process
in each semicell ; they were not comparable with lateral lobes, however.
In old cultures the aradiate form shows a number of complex cells
with a rounded middle section and more or less simplified single cells.
This indicates a sensitivity to environmental conditions.
V I . Enucleate Cells
Among the methods of procuring enucleate cells of Micrasterias,
centrifuging at metaphase has the advantage that the nuclear state can
be checked with the microscope. I t may result directly in the formation
of binucleate and enucleate daughter cells, or else in the formation of
double cells, with a nucleate and an enucleate isthmus, which during the
subsequent divisions may produce enucleate cells at the enucleate isthmus (Section V I I ; Kallio, 1957, 1959). Enucleate cells have been
obtained from the normal cells of various species and from the
uniradiate, aradiate and triradiate cells. In the case of single cells, it
is, in fact, a parent semicell that becomes enucleated, but in all cases
it is capable of forming a new semicell which, though extremely simplified in shape, reveals the radiation of the parent semicell, i.e. the
number of wings is retained (Fig. 3). This fact is of particular
theoretical significance.
Under normal culture conditions the enucleate cells only survive for
about 24 h, but their life can be prolonged considerably by increasing
the calcium ion concentration, particularly at low temperatures (Waris,
1951). Their rapid death is caused by the bursting of the new cell wall,
which is very weak and probably formed of material rich in pectic
substances but low in cellulose. This composition would account for the
beneficial effect of calcium.
The short survival of the enucleate Micrasterias cells might seem
surprising in view of the extremely long life (up to many months) of the
enucleate sections of Acetabularia cells studied by Haemmerling and his
school (latest review 1963). In Acetabularia the longevity is regarded as
due to the accumulation in the cell of nuclear substances concerned with
the synthesis of cytoplasmic ribonucleic acid (UNA). Probably such
substances are not present in the enucleate Micrasterias cells, or are
almost negligible in amount. This conclusion is based on the difference
in the effects of ultraviolet irradiation between Micrasterias and Acetabularia, In the enucleate cells of Micrasterias, the effect of ultraviolet
MICRASTERIAS
55
radiate form of this species about twenty in every thousand divisions
resulted in the formation of the normal biradiate form. This shows that
the aradiate form is at all events much more stable than the uniradiate
one. The only sign of a lateral differentiation so far seen was represented by a single cell among thousands which had a spine-like process
in each semicell ; they were not comparable with lateral lobes, however.
In old cultures the aradiate form shows a number of complex cells
with a rounded middle section and more or less simplified single cells.
This indicates a sensitivity to environmental conditions.
V I . Enucleate Cells
Among the methods of procuring enucleate cells of Micrasterias,
centrifuging at metaphase has the advantage that the nuclear state can
be checked with the microscope. I t may result directly in the formation
of binucleate and enucleate daughter cells, or else in the formation of
double cells, with a nucleate and an enucleate isthmus, which during the
subsequent divisions may produce enucleate cells at the enucleate isthmus (Section V I I ; Kallio, 1957, 1959). Enucleate cells have been
obtained from the normal cells of various species and from the
uniradiate, aradiate and triradiate cells. In the case of single cells, it
is, in fact, a parent semicell that becomes enucleated, but in all cases
it is capable of forming a new semicell which, though extremely simplified in shape, reveals the radiation of the parent semicell, i.e. the
number of wings is retained (Fig. 3). This fact is of particular
theoretical significance.
Under normal culture conditions the enucleate cells only survive for
about 24 h, but their life can be prolonged considerably by increasing
the calcium ion concentration, particularly at low temperatures (Waris,
1951). Their rapid death is caused by the bursting of the new cell wall,
which is very weak and probably formed of material rich in pectic
substances but low in cellulose. This composition would account for the
beneficial effect of calcium.
The short survival of the enucleate Micrasterias cells might seem
surprising in view of the extremely long life (up to many months) of the
enucleate sections of Acetabularia cells studied by Haemmerling and his
school (latest review 1963). In Acetabularia the longevity is regarded as
due to the accumulation in the cell of nuclear substances concerned with
the synthesis of cytoplasmic ribonucleic acid (UNA). Probably such
substances are not present in the enucleate Micrasterias cells, or are
almost negligible in amount. This conclusion is based on the difference
in the effects of ultraviolet irradiation between Micrasterias and Acetabularia, In the enucleate cells of Micrasterias, the effect of ultraviolet