|dc.description.abstract||Two species of Euglena, Euglena gracilis and Eug1ena mutabilis,
were cultured with the object of finding the conditions which would
give the highest mitotic index at a predictable time. This was done,
and the cell-cycle time under controlled conditions was found. Following
this, mitosis was investigated by ordinary cytological methods and with
the electron microscope. Using the microdensitometer, it was found that
DNA replication occurred during interphase and in other organisms.
Evidence for the presence of separated chromatids was found at the end
of interphase (G2).
Counting sectioned chromatin masses in sections photographed under
the electron microscope showed the presence of separated chromatids at
metaphase. It was found that irradiation made metaphase and particularly
anaphase stages easily spread, and from these stages it was seen that the
chromatid is characteristically V-shaped with the point of the V directed
to the poles. At metaphase however the point was predominantly on the
Detailed study of the ultrastructure of the mitotic figures showed
that microtubules are connected by arms or bridges; they are also
attached laterally to the endosome and probably to the chromatin; in
addition they pass through the chromatin. The relationship between
chromosomes and microtubules has been followed by means of aerial
transverse sections and in 1ongitudinal sections. Models for mitosis in
Euglena have been devised.
The ultrastructure of the endosome has been investigated and
observations suggesting the presence within it a nucleolar-organising
chromosomal region, have been made.
The effects of irradiation have been investigated using gamma
radiation in doses from 4Kr to 350Kr. A dose-tolerance curve has been
plotted. LD.50 is 35Kr. The effects of radiation on the ultrastructure
of the nucleus are described.||en_US