VI CELL-DIVISION 67 



as their attendant astrospheres, being formed by the binary 

 fission of those of the vwther-nudeiis. 



But pari passu with the process of nuclear division, 

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

 place by a simple process of constriction (h)— 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 as a row of granules formed from the equatorial part 

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

 form a complete equatorial plate dividing the cell-body into 

 two halves : fission then takes place by the cell-plate split- 

 ting into two along a plane parallel with its flat surfaces.* 

 In plants the cell-plate gives rise to a partition wall of 

 cellulose which divides the two daughter-cells from one 

 another. 



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 mitosis or karyokinesis is 

 applied : direct division is then distinguished as amitotic. 



In this connection the reader will not fail to note the 

 extreme complexity of structure revealed in cells and their 

 nuclei by the highest powers of the microscope. When the 

 constituent cells of the higher animals and plants were 

 discovered, during the early years of the present century, by 

 Schleiden and Schwann, they were looked upon as the ultima 

 Thule of microscopic analysis. Now the demonstration of 



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

 sented in such diagrams as Fig. 10 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 2 



