REPRODUCTION 415 



into two places which are determined by the positions of 

 the poles of the nucleus, or of the centrosomes 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 denned 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. 



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 does not contain a relatively 

 large vacuole and 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. 165, 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. 



If the protoplast has a large central vacuole, the spindle 

 often 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. 



In some of the Thallophytes the new wall is formed 

 without the intervention of a spindle. After the two new 



