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Nuclear division in Oxyrrhis marina, Duj. 217 
XVII.—Nuclear division in the Dinoflagellate, Oxyrrhis marina, Duj. 
By J. S. Dunkerly, B.Sc., Lecturer in Zvology, University of 
Glasgow. 
(With Text-figures.) 
(Read 25th October 1920. Received 8th November 1920.) 
Tae following account of the nuclear division in Oxyrrhis marina, Duj., is 
based on some preparations made from laboratory cultures grown in sea 
water containing Allen and Nelson’s modification of Miquel’s nutritive 
solution. Coverslips were floated on the surface of the culture fluid and 
left for twenty-four hours, then fixed in Bouin’s fluid and stained by 
Dobell’s method with alcoholic iron hematoxylin. Many different stages 
of division in Oxyrrhis marina were seen, but the following stages have 
been selected as showing a probable sequence. 
The resting nucleus is ovoid with a central body which may be called 
the karyosome, usually difficult to see on account of the deeply staining 
chromatin granules which mask it. The first appearance of division is 
shown by the appearance of two karyosomes (Fig. 1), the actual method 
of their formation not being seen. The chromatin now shows an indistinct 
fibrillar appearance, and the bulky equatorial plate is arranged at right 
angles to the long axis of the cell. Fig. 2 shows this at a slightly later 
stage when the chromatin is just dividing into the two daughter plates 
and two karyosomes are distinctly seen. At the two ends of the spindle 
can be seen. the two faintly staining polar caps, comparable with those 
described by Ford (2) in Amwba tachypodia but larger, as would be expected 
from the larger volume of the chromatin mass in Oxyrrhis marina. In 
Fig. 5 the daughter plates have separated, the surfaces facing each other 
showing ragged threads of chromatin. Most important is the behaviour of 
the karyosomes at this stage. They appear to be connected by a distinct 
central strand or axis, in spite of the fact that two separate karyosomes 
are seen in earlier stages (compare Figs. 1 and 2). The expanded ends 
of the central axis are embedded partly in the concave sides of the daughter 
plates. The polar caps on the outer convex sides of the daughter plates 
are now more definitely conical in shape, and the whole figure shows 
a characteristic appearance of mitosis, except that the thick central axis 
takes the place of the numerous spindle fibres which would be seen in 
an ordinary mitotic figure. — 
The daughter plates on separating further have at first their concave 
