MORPHOGENESIS IN
MICRASTERIAS
11
work put forward by Waris (1950 a, b) and Kallio (1951). Since then the
hypothesis has found support in so many respects that it can now be
presented as a theory. In general form, the theory implies that the
bilateral symmetry characteristic of the normal cells is determined by a
cytoplasmic framework comprising some basic units which in cell
division are transmitted from the parental to the new semicell and are
thus responsible for the continuity of symmetry in the offspring. As to
the details, the basic units are thought to be represented by three axes,
one of which corresponds to the longitudinal median region with the
polar lobes and the other two to the lateral wings. In the new semicells,
the degree of differentiation depends on nuclear action, which causes a
branching of the basic units. If a parent semicell fails to extend one or
both of the lateral axes into the new semicell, a defective form lacking
one or both of the lateral wings will result. These defective forms are
represented by the uniradiate and aradiate faciès.
To the assumption of three cytoplasmic axes the objection can be
raised that they do not appear in the electron-microscopic photographs
taken of young semicells (Waddington, 1962). Obviously the term
'axis' cannot be taken literally in the sense of distinct linear organelles,
but if it is used as a symbol for a system of fibrils it may be applicable.
The electron-microscopic photographs concerned reveal a certain degree
of radial arrangement in the cytoplasmic structure which does not seem
to exclude the possibility of persistent fibrils. On the other hand, for
the postulated axes some evidence has been found in the cytoplasmic
strands which can be seen in young semicells subjected to the influence
of a strong glucose solution (Section X).
Irrespective of the possible nature of the 'axes', it remains a fact that
in their capacity to appear in the form of the biradiate, uniradiate and
aradiate faciès the Micrasterias species behave as if their cell comprised
three relatively autonomous sections, one corresponding to the median
region with the polar lobes and two corresponding to the lateral wings.
Any attempt to explain the morphogenesis in Micrasterias should take
this experimental fact into consideration.
Teiling (1950) expressed the opinion that the actual differentiation
takes place in the outermost layer of the cytoplasm, which he called
'meristematic'. This layer was supposed to include certain cytoplasmic
organelles which undergo differentiation into secondary organelles
and so on as the young semicell develops, but it was not directly stated
whether the organelles were thought to be transmitted to the offspring.
Teiling also supposed that the radiation, i.e. the number of wings, is
already determined at the vesicle stage of the young semicells.
In agreement with Teiling, Kiermayer and Jarosch (1962) and Jarosch
and Kiermayer (1962) consider the differentiation in Micrasterias rotata
MICRASTERIAS
11
work put forward by Waris (1950 a, b) and Kallio (1951). Since then the
hypothesis has found support in so many respects that it can now be
presented as a theory. In general form, the theory implies that the
bilateral symmetry characteristic of the normal cells is determined by a
cytoplasmic framework comprising some basic units which in cell
division are transmitted from the parental to the new semicell and are
thus responsible for the continuity of symmetry in the offspring. As to
the details, the basic units are thought to be represented by three axes,
one of which corresponds to the longitudinal median region with the
polar lobes and the other two to the lateral wings. In the new semicells,
the degree of differentiation depends on nuclear action, which causes a
branching of the basic units. If a parent semicell fails to extend one or
both of the lateral axes into the new semicell, a defective form lacking
one or both of the lateral wings will result. These defective forms are
represented by the uniradiate and aradiate faciès.
To the assumption of three cytoplasmic axes the objection can be
raised that they do not appear in the electron-microscopic photographs
taken of young semicells (Waddington, 1962). Obviously the term
'axis' cannot be taken literally in the sense of distinct linear organelles,
but if it is used as a symbol for a system of fibrils it may be applicable.
The electron-microscopic photographs concerned reveal a certain degree
of radial arrangement in the cytoplasmic structure which does not seem
to exclude the possibility of persistent fibrils. On the other hand, for
the postulated axes some evidence has been found in the cytoplasmic
strands which can be seen in young semicells subjected to the influence
of a strong glucose solution (Section X).
Irrespective of the possible nature of the 'axes', it remains a fact that
in their capacity to appear in the form of the biradiate, uniradiate and
aradiate faciès the Micrasterias species behave as if their cell comprised
three relatively autonomous sections, one corresponding to the median
region with the polar lobes and two corresponding to the lateral wings.
Any attempt to explain the morphogenesis in Micrasterias should take
this experimental fact into consideration.
Teiling (1950) expressed the opinion that the actual differentiation
takes place in the outermost layer of the cytoplasm, which he called
'meristematic'. This layer was supposed to include certain cytoplasmic
organelles which undergo differentiation into secondary organelles
and so on as the young semicell develops, but it was not directly stated
whether the organelles were thought to be transmitted to the offspring.
Teiling also supposed that the radiation, i.e. the number of wings, is
already determined at the vesicle stage of the young semicells.
In agreement with Teiling, Kiermayer and Jarosch (1962) and Jarosch
and Kiermayer (1962) consider the differentiation in Micrasterias rotata