ABSTRACT
After individual cytoplasmic lobes of growing hemicells of Micrasterias were irradiated for 30 or 60 s with a microbeam of ultraviolet light 20 in diameter, the corresponding parts of the fully developed wings were stunted. Tn subsequent divisions the new hemicells were normal.
In one case a hemicell, grossly malformed as a result of a 5 min dose of ultraviolet light to the cytoplasm, did not give a completely normal hemicell in the first seven divisions after treatment, although it did so in subsequent divisions.
Abnormal development was induced with dinitrophenol.
The ultraviolet microbeam irreversibly inactivated the nucleus when applied to it for longer than 5 s. Growth continued after nuclear inactivation but neither thickening of the cell walls nor differentiation took place.
After applying the microbeam to the nucleus for 3-4 s, an anuclear type of development was followed by cell-wall thickening without differentiation. Subsequent divisions took place normally.
After applying the microbeam to the nucleus for 2 s, partial differentiation took place and subsequent divisions were normal.
By allowing hemicells to develop in the presence of actinomycin D at high concentrations for short periods, exactly the same patterns of abnormal development followed by cell recovery were observed as after ultraviolet irradiation of the nucleus.
From the experiments, estimates were made of the times of nuclear determinations of lobe-splitting, cell-wall thickening and differentiation. Nuclear determination appeared to take place between 30 and 155 min before the developmental event was observed under the light microscope.
