76
H. W A M S AND P . KALLIO
The branching of the cytoplasmic axes is favoured by an increase in
the nuclear influence per cytoplasmic unit. This is the theoretical
interpretation of the fact that the area of the lobes and the number of
lobules may increase if the nuclear volume increases, as in the polyploid
cells, or if the cytoplasmic volume decreases, as in the uniradiate cells.
In both cases a duplication of the lateral lobes or their parts on two
levels is a common phenomenon which is always accompanied by the
formation of gaps adjacent to the duplicated parts. In structure the
semicell may then be likened to a fan in which a partial folding causes
gaps between the ribs. I t may be pointed out that in the case of this
type of lobe duplication the number of lobules and dents remains about
the same as when all the lobules have developed on the same level.
Thus the duplication, in fact, involves a rearrangement of existing
units. In theoretical terms, there is a definite number of cytoplasmic
units; if some of them overlap, gaps must arise.
Kallio (1951, 1953a, b, 1959, 1960) has elaborated the concept of
'nuclear effect per cytoplasmic unit'. This implies that it is not the
nuclear effect on the cytoplasm as a whole that matters but the effect
relative to the number of structural units which function as growth
centres.
The theory put forward in this section will be compared with others
in the concluding section.
X I I . Conclusions
The special advantage offered by the genus Micrasterias for the study
of morphogenetic problems is due to the fact that the symmetrical
structure of the cells, characteristic of the desmids, may change in
response to external and internal factors and so serve to elucidate the
roles of nucleus and cytoplasm in differentiation. Thus it has been
possible to produce cells defective in symmetry, and polyploid, aneuploid, enucleate and complex cells. Viable clones have been derived
from the defective, polyploid and aneuploid cells. Complex cells, composed of two or more single cells lacking septa, have provided an
exceptional opportunity for studying the interactions of nucleus and
cytoplasm and of different nuclei. These interactions involve problems
such as the significance for differentiation of the relative masses of
nucleus and cytoplasm, the significance of the distance between the
nucleus and the growth centre in the cell, the synchronization of the
divisions of two nuclei with different chromosome complements in the
same cell, the effect of a healthy nucleus on one injured by irradiation,
and the ageing of the cell.
The occurrence of the uniradiate defective form of Micrasterias
thomasiana var. notata led to the hypothesis of the cytoplasmic frame-
H. W A M S AND P . KALLIO
The branching of the cytoplasmic axes is favoured by an increase in
the nuclear influence per cytoplasmic unit. This is the theoretical
interpretation of the fact that the area of the lobes and the number of
lobules may increase if the nuclear volume increases, as in the polyploid
cells, or if the cytoplasmic volume decreases, as in the uniradiate cells.
In both cases a duplication of the lateral lobes or their parts on two
levels is a common phenomenon which is always accompanied by the
formation of gaps adjacent to the duplicated parts. In structure the
semicell may then be likened to a fan in which a partial folding causes
gaps between the ribs. I t may be pointed out that in the case of this
type of lobe duplication the number of lobules and dents remains about
the same as when all the lobules have developed on the same level.
Thus the duplication, in fact, involves a rearrangement of existing
units. In theoretical terms, there is a definite number of cytoplasmic
units; if some of them overlap, gaps must arise.
Kallio (1951, 1953a, b, 1959, 1960) has elaborated the concept of
'nuclear effect per cytoplasmic unit'. This implies that it is not the
nuclear effect on the cytoplasm as a whole that matters but the effect
relative to the number of structural units which function as growth
centres.
The theory put forward in this section will be compared with others
in the concluding section.
X I I . Conclusions
The special advantage offered by the genus Micrasterias for the study
of morphogenetic problems is due to the fact that the symmetrical
structure of the cells, characteristic of the desmids, may change in
response to external and internal factors and so serve to elucidate the
roles of nucleus and cytoplasm in differentiation. Thus it has been
possible to produce cells defective in symmetry, and polyploid, aneuploid, enucleate and complex cells. Viable clones have been derived
from the defective, polyploid and aneuploid cells. Complex cells, composed of two or more single cells lacking septa, have provided an
exceptional opportunity for studying the interactions of nucleus and
cytoplasm and of different nuclei. These interactions involve problems
such as the significance for differentiation of the relative masses of
nucleus and cytoplasm, the significance of the distance between the
nucleus and the growth centre in the cell, the synchronization of the
divisions of two nuclei with different chromosome complements in the
same cell, the effect of a healthy nucleus on one injured by irradiation,
and the ageing of the cell.
The occurrence of the uniradiate defective form of Micrasterias
thomasiana var. notata led to the hypothesis of the cytoplasmic frame-