126
Persidsky,
T h e sexual process in Melosira varians.
The stage of late anaphase of the heterotypic division, has been
encountered very often. Fig. 5 illustrates the moment when the
chromosomes, forming a dense ring, have moved to the poles,
whilst the spindle filaments are distinctly seen between them.
At this moment, the valves, which, up till now, have remained
closely pressed to one another, begin to separate, the protopla t
grows in size, and there appeares in it a single large vacuo! .
As a result of the first division, we have in the cell of M. varians
two nuclei (fig. 6), with a distinctly perceivable membrane, with
scarcely distinguishable grains of chromatin adhering to its inni r
side. Fig. 7 shows the chromatin substance as yet unaltered, and
both the nuclei appear homogenous. In both cases, the nucleoli
do not present their usual aspect, but consist of separate grains,
staining intensely, disposed in a dense ring, the number of grairs
coinciding with the number of chromosomes. The minute dimen­
sions of my object did not permit of my tracing the formation
process of the nucleolus, but I consider it necessary to point out
the close connection between the nucleolus and the chromosome,
similar to the connection which P. S c h m i d t observed in
Coscinodiscus.
The binuclear stage is not a very protracted one, and very
soon, the homeotypic division begins in the nuclei, taking place
simultaneously in both, or, sometimes, somewhat earlier in one
of them. On fig. 8, two nuclei in the spirem stage are seen with
the characteristic disposition of the chromatin filaments along the
periphery. In the hetereotypical division, as well as in the homeotypical one, I could not detect some of the stages. For instance,
the very important metaphase was not found a single time, and
only anaphase and telophase stages pervaded my preparations.
The anaphase of the homeotypic division differs from the hetero­
typic one by the presence of an intranuclear spindle, the nuclear
membrane existing up to the moment, when the chromosomes
move to the poles (fig. 9). As the chromatin begins to form, the
entire figure becomes biscuit-shaped, and finally, the connection
between the nuclei disappearing, there are, as a result, four nuclei in
the cell of M. varians (fig. 11—14). Sometimes, as above mentioned,
the division of one nucleus lags behind that of the other; this
happens somewhat frequently and is represented on fig. 10.
Soon after the close of the homeotypic division, the four
nuclei do not yet possess any definite structure; their mem­
brane is exceedingly thin, the chromosomes, oftenest six in
number, very much reduced in size, are scattered without any
order, and there is no nucleolus. No essential changes could be
established in the protoplast, as compared to the binuclear
stage. Later on, the reduced chromosomes begin to form a ring,
loosing their individuality, while the nucleus begins to stain
more vividly, though still remaining deprived of any structure.
The protoplast continues to growin size. The shape of the
chromatophores in optic section remains oval, but their size
increases somewhat (fig. 12—13).