MORPHOGENESIS IN
MICRASTERIAS
75
although they grow under nuclear influence. In this sense the basis of
the bilateral symmetry is independent of the nucleus, whereas the
degree of differentiation depends on the nucleus. This explanation of the
morphogenesis in Micrasterias was first suggested by the occurrence and
behaviour of the uniradiate faciès of M. thomasiana var. notata (Waris,
1950b; Kallio, 1951), but it has since then found further support from
the results of new experiments. The main arguments in favour of the
theory are as follows :
(a) The mode of origination of the uniradiate faciès makes it improbable that a nuclear mutation is concerned.
(b) The uniradiate form has been transmitted to offspring through
innumerable cell divisions, although reversion to the normal form has
taken place. In successive cell divisions the defect always appears on the
same side of the cell.
(c) An aradiate faciès, lacking the lateral lobes on both sides, has been
obtained from Micrasterias americana. In clone culture it has not so far
been seen to revert to the normal form.
(d) On enucleation, as a rule, a normal biradiate parent semicell forms
a new semicell which, though of simple shape, is biradiate too, while a
uniradiate parent semicell forms a uniradiate new semicell with the
defect on the same side. An aradiate parent semicell forms an aradiate
new semicell.
Though a rare event, the uniradiate form has arisen several times in
the cultures of M. thomasiana and, in addition, in the cultures of M.
rotata var. evoluta, M. torreyi, M. fimbriata and M. americana. This fact
and the appearance of the aradiate form of M. americana suggest that a
normal Micrasterias cell comprises three main structural units, one
corresponding to the longitudinal median section with the polar lobes
and two corresponding to the lateral lobes. It is obvious that the median
section cannot be omitted because it contains the nucleus.
According to the theory, the three main sections of the cell include
three cytoplasmic 'axes' which are thought to consist of fibrils (Waris,
1950b; Kallio, 1951). In cell division the axes become severed at the
isthmus into two semi-axes which, in normal development, grow from
the parent into the new semicell and regenerate their missing halves.
The differentiation of the semicell into lobes presupposes a branching
of the axes. If for some reason a parent semicell fails to extend a lateral
axis into the new semicell, the corresponding lobe will be lacking. On
the other hand, the reversion of the uniradiate to the biradiate form is
presumed to be due to branching of the one lateral axis present. If this
explanation is correct, the absence of both lateral wings should represent an irreversible situation, which, to judge from the present evidence,
is realized in the aradiate form of M. americana.
MICRASTERIAS
75
although they grow under nuclear influence. In this sense the basis of
the bilateral symmetry is independent of the nucleus, whereas the
degree of differentiation depends on the nucleus. This explanation of the
morphogenesis in Micrasterias was first suggested by the occurrence and
behaviour of the uniradiate faciès of M. thomasiana var. notata (Waris,
1950b; Kallio, 1951), but it has since then found further support from
the results of new experiments. The main arguments in favour of the
theory are as follows :
(a) The mode of origination of the uniradiate faciès makes it improbable that a nuclear mutation is concerned.
(b) The uniradiate form has been transmitted to offspring through
innumerable cell divisions, although reversion to the normal form has
taken place. In successive cell divisions the defect always appears on the
same side of the cell.
(c) An aradiate faciès, lacking the lateral lobes on both sides, has been
obtained from Micrasterias americana. In clone culture it has not so far
been seen to revert to the normal form.
(d) On enucleation, as a rule, a normal biradiate parent semicell forms
a new semicell which, though of simple shape, is biradiate too, while a
uniradiate parent semicell forms a uniradiate new semicell with the
defect on the same side. An aradiate parent semicell forms an aradiate
new semicell.
Though a rare event, the uniradiate form has arisen several times in
the cultures of M. thomasiana and, in addition, in the cultures of M.
rotata var. evoluta, M. torreyi, M. fimbriata and M. americana. This fact
and the appearance of the aradiate form of M. americana suggest that a
normal Micrasterias cell comprises three main structural units, one
corresponding to the longitudinal median section with the polar lobes
and two corresponding to the lateral lobes. It is obvious that the median
section cannot be omitted because it contains the nucleus.
According to the theory, the three main sections of the cell include
three cytoplasmic 'axes' which are thought to consist of fibrils (Waris,
1950b; Kallio, 1951). In cell division the axes become severed at the
isthmus into two semi-axes which, in normal development, grow from
the parent into the new semicell and regenerate their missing halves.
The differentiation of the semicell into lobes presupposes a branching
of the axes. If for some reason a parent semicell fails to extend a lateral
axis into the new semicell, the corresponding lobe will be lacking. On
the other hand, the reversion of the uniradiate to the biradiate form is
presumed to be due to branching of the one lateral axis present. If this
explanation is correct, the absence of both lateral wings should represent an irreversible situation, which, to judge from the present evidence,
is realized in the aradiate form of M. americana.