MORPHOGENESIS IN
MICEASTERIAS
73
the earlier finding that there is a positive correlation between the nuclear quantity and the degree of differentiation of the cell in Micrasterias.
The ultraviolet irradiation experiments have also shown that there is
also another morphological control in the cell which is insensitive to
ultraviolet irradiation. The radiation and the degree of symmetry, i.e.
the number of the wings, is always preserved after the treatment of the
whole cell. The new semicells of three-winged, two-winged, one-winged
and wingless cells have the same degrees of symmetry as the parent
semicells (Fig. 3). The growth after the exposure to a lethal dose of
irradiation is not gene-controlled. Growth does, however, continue and
some of the typical features of the genus Micrasterias are still in evidence. This is determined by a plasmatic factor, which is capable of
reproduction and which transmits the same characteristics to the
daughter semicell during cytokinesis.
X . Effects of Chemical Compounds
A. Effects of Weak Acids on the Nucleoli
At interphase the nucleolus of many species is represented by a
number of semisolid, irregular particles (the 'multiple nucleolus'). At
prophase these dissolve and partly fuse to form a group of spherical
bodies suggestive of a cluster of grapes; hence the term 'grape stage'
(Waris, 1950a). This 'grape stage' can also be induced artificially by
subjecting cells at interphase to the influence of weak acids (Waris,
1956). It seems possible, therefore, that the induction of prophase involves an acidification of the cell, the impulse for nuclear division
coming from the cytoplasm. A grape stage with a very reduced number
of nucleolar bodies (even a single one) has often been observed in cells
taken from, old cultures ; it seems to be a consequence of inhibited
reproduction and may well be connected with the accumulation of some
acidic substances.
B. Effects on the Development of New Semicells
A distinction can be made between the osmotic effects of high concentrations in general and the specific effects of definite chemical
compounds. However, it is not always possible to make a sharp distinction between these effects.
If Micrasterias cells with young growing semicells are subjected to
the influence of high glucose concentrations, which lower the turgor
pressure, the expansion of the new semicells is more or less inhibited
and the new cell walls thicken by the deposition of new membrane
substance. Jarosch and Kiermayer (1962) and Kiermayer and Jarosch
(1962) have directed attention to the uneven deposition of membrane
MICEASTERIAS
73
the earlier finding that there is a positive correlation between the nuclear quantity and the degree of differentiation of the cell in Micrasterias.
The ultraviolet irradiation experiments have also shown that there is
also another morphological control in the cell which is insensitive to
ultraviolet irradiation. The radiation and the degree of symmetry, i.e.
the number of the wings, is always preserved after the treatment of the
whole cell. The new semicells of three-winged, two-winged, one-winged
and wingless cells have the same degrees of symmetry as the parent
semicells (Fig. 3). The growth after the exposure to a lethal dose of
irradiation is not gene-controlled. Growth does, however, continue and
some of the typical features of the genus Micrasterias are still in evidence. This is determined by a plasmatic factor, which is capable of
reproduction and which transmits the same characteristics to the
daughter semicell during cytokinesis.
X . Effects of Chemical Compounds
A. Effects of Weak Acids on the Nucleoli
At interphase the nucleolus of many species is represented by a
number of semisolid, irregular particles (the 'multiple nucleolus'). At
prophase these dissolve and partly fuse to form a group of spherical
bodies suggestive of a cluster of grapes; hence the term 'grape stage'
(Waris, 1950a). This 'grape stage' can also be induced artificially by
subjecting cells at interphase to the influence of weak acids (Waris,
1956). It seems possible, therefore, that the induction of prophase involves an acidification of the cell, the impulse for nuclear division
coming from the cytoplasm. A grape stage with a very reduced number
of nucleolar bodies (even a single one) has often been observed in cells
taken from, old cultures ; it seems to be a consequence of inhibited
reproduction and may well be connected with the accumulation of some
acidic substances.
B. Effects on the Development of New Semicells
A distinction can be made between the osmotic effects of high concentrations in general and the specific effects of definite chemical
compounds. However, it is not always possible to make a sharp distinction between these effects.
If Micrasterias cells with young growing semicells are subjected to
the influence of high glucose concentrations, which lower the turgor
pressure, the expansion of the new semicells is more or less inhibited
and the new cell walls thicken by the deposition of new membrane
substance. Jarosch and Kiermayer (1962) and Kiermayer and Jarosch
(1962) have directed attention to the uneven deposition of membrane