MORPHOGENESIS IN MICRASTERIAS
51
ticularly EDTA, is desirable (Waris, 1953). The temperature should not
rise much above 20° C (Waren, 1926). For some species a rather lower
temperature, about 17° C, is more suitable. The illumination can be
arranged with fluorescent tubes. For morphogenetic experiments it is of
significance that nuclear divisions can be induced to take place mainly
within certain hours during the day-time if the cultures are illuminated
at night, whereas with continuous illumination divisions may occur at
any time. Attention must be called to the fact that strong uninterrupted
illumination is injurious and may cause abnormal development of
the cells. On the other hand, such conditions can be employed to
produce complex cells, which have proved to be of particular theoretical
interest.
For the handling of the cells a dissecting microscope is desirable. The
cells can be transferred with glass capillaries and their position adjusted
with fine glass needles, with which the mucilage can also be removed in
order to keep the cells immobile for observation. For this purpose it is
often necessary to repeat the removal of the mucilage from time to time.
For observation the cells may be put in a watch-glass provided with
nutrient solution. If the cells are to be treated at a definite nuclear stage,
single cells must be observed with a water-immersion objective; a
magnification of x 40 is suitable. It requires a great deal of experience
to recognize the nucleolar states characteristic of the nuclear stages.
Once they are recognized, it is possible to predict the onset of nuclear
division in sufficient time to perform treatment at the right nuclear
stage. For instance, polyploid, aneuploid, complex and enucleate cells
can be produced by centrifugation at metaphase. To achieve results,
single cells must be centrifuged in the direction of the longitudinal axis
for a suitable length of time. The details of the procedure have been
described by Waris (1950b) and Kallio (1951). For this purpose centrifuge tubes must be prepared which hold single cells in the correct
position; the tubes must have flat and narrow ends, and it must be
possible to check the effect of centrifugation at short intervals.
V. Persistent Defects in Form
In the clone cultures of several Micrasterias species defective forms
have arisen which have persisted through a long or indefinite succession
of cell divisions. The defect appears in the lack of one or both of the two
lateral wings, according to which the forms are called respectively
uniradiate and aradiate. The former is reversible, while the latter has
not been seen to revert to the normal form. The occurrence of these
forms Jias proved to be of particular significance for the theory of
morphogenesis in Micrasterias,
51
ticularly EDTA, is desirable (Waris, 1953). The temperature should not
rise much above 20° C (Waren, 1926). For some species a rather lower
temperature, about 17° C, is more suitable. The illumination can be
arranged with fluorescent tubes. For morphogenetic experiments it is of
significance that nuclear divisions can be induced to take place mainly
within certain hours during the day-time if the cultures are illuminated
at night, whereas with continuous illumination divisions may occur at
any time. Attention must be called to the fact that strong uninterrupted
illumination is injurious and may cause abnormal development of
the cells. On the other hand, such conditions can be employed to
produce complex cells, which have proved to be of particular theoretical
interest.
For the handling of the cells a dissecting microscope is desirable. The
cells can be transferred with glass capillaries and their position adjusted
with fine glass needles, with which the mucilage can also be removed in
order to keep the cells immobile for observation. For this purpose it is
often necessary to repeat the removal of the mucilage from time to time.
For observation the cells may be put in a watch-glass provided with
nutrient solution. If the cells are to be treated at a definite nuclear stage,
single cells must be observed with a water-immersion objective; a
magnification of x 40 is suitable. It requires a great deal of experience
to recognize the nucleolar states characteristic of the nuclear stages.
Once they are recognized, it is possible to predict the onset of nuclear
division in sufficient time to perform treatment at the right nuclear
stage. For instance, polyploid, aneuploid, complex and enucleate cells
can be produced by centrifugation at metaphase. To achieve results,
single cells must be centrifuged in the direction of the longitudinal axis
for a suitable length of time. The details of the procedure have been
described by Waris (1950b) and Kallio (1951). For this purpose centrifuge tubes must be prepared which hold single cells in the correct
position; the tubes must have flat and narrow ends, and it must be
possible to check the effect of centrifugation at short intervals.
V. Persistent Defects in Form
In the clone cultures of several Micrasterias species defective forms
have arisen which have persisted through a long or indefinite succession
of cell divisions. The defect appears in the lack of one or both of the two
lateral wings, according to which the forms are called respectively
uniradiate and aradiate. The former is reversible, while the latter has
not been seen to revert to the normal form. The occurrence of these
forms Jias proved to be of particular significance for the theory of
morphogenesis in Micrasterias,