78
H. WAMS AND P. KALLIO
to be due to phenomena in the periphery of the young semicell. These
two authors emphasize the formative significance of the uneven
deposition of membrane substance and introduce the concept of the
'initial deposition pattern' of the membrane. They reject the idea of the
cytoplasmic framework as not solving the problem but only deferring
it, and consider the possibility that some kind of 'stationary waves'
('stehende Wellen') with interference phenomena might be the ultimate
reason for differentiation.
Here we come to the same question that Waddington (1962) has
touched upon in referring to the possibility that in the flowing cytoplasm of the young Micrasterias cell a dynamic equilibrium might set
in, determining differentiation. This line of thought, as well as the
concept of the 'stationary waves', seems to refer the differentiation
problem to a molecular level. From the point of view supported by the
present authors the existence of a dynamic equilibrium need not be
denied, but on this basis it seems difficult to explain the occurrence and
behaviour of the uniradiate and aradiate forms. In general, it does not
yet seem possible to explain differentiation at a molecular level, as
Steward and Moham Ram (1961) have also stated in dealing with the
factors determining the morphogenesis of plants. Thus, in our opinion,
differentiation must still be explained on the basis of pre-existing cellular
structures.
Even if the significance of pre-existing cellular structures is conceded,
the question of their location in the cell may require further consideration. We assume an axial position of the fibrillar structures determining
differentiation in Micrasterias, whereas Green (1962a, b, 1963) suggests
that in Nitella and Bryopsis the synthesis of new cell wall structures is
controlled by cytoplasmic fibrils orientated in the periphery of the cell.
Green, like Kiermayer and Jarosch, regards the expansion of the cell as
due to the yielding of the cell wall to the turgor pressure of the vacuole.
In the case of Micrasterias, it seems that the duplication of the lobes
accompanied by the formation of gaps is better accounted for by the
assumption of an axial system of cytoplasmic fibrils. But a parietal
position of the fibrils, particularly in the later stages of development,
is not impossible, although in the initial stages, just after the formation
of the septum, a central position seems more likely.
If we try to relate the differentiation of the new semicell to phenomena
confined to the periphery, the uniradiate facies presents particular
difficulty. In a series of successive cell divisions a line can be followed in
which at each division the preceding new semicell functions as the parent
semicell. The defect will then appear alternately at two opposite ends of
the same longitudinal section of the cell. If we reject the assumption of
cytoplasmic axes it remains to be explained in what way the one-sided
H. WAMS AND P. KALLIO
to be due to phenomena in the periphery of the young semicell. These
two authors emphasize the formative significance of the uneven
deposition of membrane substance and introduce the concept of the
'initial deposition pattern' of the membrane. They reject the idea of the
cytoplasmic framework as not solving the problem but only deferring
it, and consider the possibility that some kind of 'stationary waves'
('stehende Wellen') with interference phenomena might be the ultimate
reason for differentiation.
Here we come to the same question that Waddington (1962) has
touched upon in referring to the possibility that in the flowing cytoplasm of the young Micrasterias cell a dynamic equilibrium might set
in, determining differentiation. This line of thought, as well as the
concept of the 'stationary waves', seems to refer the differentiation
problem to a molecular level. From the point of view supported by the
present authors the existence of a dynamic equilibrium need not be
denied, but on this basis it seems difficult to explain the occurrence and
behaviour of the uniradiate and aradiate forms. In general, it does not
yet seem possible to explain differentiation at a molecular level, as
Steward and Moham Ram (1961) have also stated in dealing with the
factors determining the morphogenesis of plants. Thus, in our opinion,
differentiation must still be explained on the basis of pre-existing cellular
structures.
Even if the significance of pre-existing cellular structures is conceded,
the question of their location in the cell may require further consideration. We assume an axial position of the fibrillar structures determining
differentiation in Micrasterias, whereas Green (1962a, b, 1963) suggests
that in Nitella and Bryopsis the synthesis of new cell wall structures is
controlled by cytoplasmic fibrils orientated in the periphery of the cell.
Green, like Kiermayer and Jarosch, regards the expansion of the cell as
due to the yielding of the cell wall to the turgor pressure of the vacuole.
In the case of Micrasterias, it seems that the duplication of the lobes
accompanied by the formation of gaps is better accounted for by the
assumption of an axial system of cytoplasmic fibrils. But a parietal
position of the fibrils, particularly in the later stages of development,
is not impossible, although in the initial stages, just after the formation
of the septum, a central position seems more likely.
If we try to relate the differentiation of the new semicell to phenomena
confined to the periphery, the uniradiate facies presents particular
difficulty. In a series of successive cell divisions a line can be followed in
which at each division the preceding new semicell functions as the parent
semicell. The defect will then appear alternately at two opposite ends of
the same longitudinal section of the cell. If we reject the assumption of
cytoplasmic axes it remains to be explained in what way the one-sided