EEPEODUCTION 415 



which they are approaching (fig. 163, 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. 163, d, e) 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. 



