REPRODUCTION 415 
which they are approaching (fig. 168, c). They thus collect 
into two places which are determined by the positions of 
the poles of the nucleus, or of the centrospheres if the latter 
are present, and they present there the appearance of two 
somewhat star-shaped aggregations. This is known as the 
diaster stage. The chromosomes at each pole next be- 
come united by their ends, and constitute two new nuclei, 
each gradually becoming well defined by the appearance of 
a nuclear membrane; the original appearance is com- 
pleted by the development of nucleoli in each new nucleus. 
The mechanism of the movement of the chromosomes 
towards the poles is not fully understood at present, but it 
is held by some observers to be due to a contraction of the 
spindle fibres to which the chromosomes are attached. In 
the cases in which a centrosphere is present at the pole it 
takes up a position by the side of the new nucleus and 
divides into two. 
This process of karyokinesis is followed in various ways 
by the production of a cell-wall between the two nuclei, 
which completes the division of the protoplast. In the 
cases in which the latter is of comparatively small diameter, 
the spindle fibres become increased in number, and form a 
barrel-shaped body whose short diameter stretches com- 
pletely across the cell (fig. 168, d, ¢) till the spindle is in 
contact with the lateral cell-walls. Granules which have 
been floating in the cell-protoplasm are to be seen stream- 
ing along the spindle fibres till they form a plate stretching 
across the cell from wall to wall. From this plate the 
septum of cellulose and its associated substances is formed. 
In certain cases the spindle does not reach completely 
across the cell. It is then at first in contact with one side 
only, and the new wall begins to be formed there in the 
same way as in the case described. It then detaches itself 
from the part of the new-formed wall which is in contact 
with the old membrane and moves gradually across to the 
opposite side of the cell, the new wall being completed as it 
goes. The spindle then disappears. 
