CELL DIVISION 65 



it then divides into two (F), the separate segments there- 

 upon travelling to opposite poles of the spindle where they 

 unite (G). 



The spindle now elongates, carrying the two masses of 

 chromatin further away from each other (H). Around each 

 of them a membrane is formed (lower half of H) enclosing a 

 portion of protoplasm, which thus becomes the achromatin 

 or nuclear protoplasm of one of the two daughter-nuclei 

 into which the original or mother-nucleus has now com- 

 pletely divided. We thus get two completely formed nuclei 

 in a single cell. 



But pari passu with this process of nuclear division, 

 fission of the cell-body is also going on. This may take 

 place by a simple process of constriction in much the 

 same way as a lump of clay or dough would divide if a loop 

 of string were tied round its middle and then tightened or 

 by the formation of what is known as a cell-plate. This 

 arises in a row of granules formed from the equatorial part 

 of the nuclear spindle (G) : the granules extend until they 

 form a complete equatorial plate dividing the cell-body into 

 two halves (H) : fission then takes place by the cell-plate 

 splitting into two along a plane parallel with its flat surfaces. 1 



In some cases the dividing nucleus instead of going 

 through the complicated processes just described divides 

 by simple constriction. We have therefore to distinguish 

 between direct and indirect nuclear division. To the latter very 

 elaborate method the name karyokinesis is often applied. 



In plant cells many similar changes are gone through 

 during the division of the nucleus. A nuclear spindle is 



1 It must not be forgotten that the cells which are necessarily repre- 

 sented in such diagrams as Figs. 8 II as planes are really solid bodies, 

 and that consequently the cell-plate represented in the figures as a line 

 is actually a plane at right angles to the plane of the paper. 



F 



