CELL DIVISION 



107 



Concerning certain nucleated cells of the metazoa we 

 are also still in doubt. Thus, Arnold and others have 

 described simple fission or direct division in certain 

 lymphoid cells of man, but whether their observations 

 would bear the test of more improved methods of 

 examination is not yet determined. The more perfect 

 the methods of staining and examining the cells, the 

 more strongly we become .convinced that there is no 

 cell division independent of antecedent changes analo- 

 gous to karyokinesis. 



When the cell divides into many segments, the karyo- 

 kinetic process is of necessity modified to conform to the 

 requirements of the case. The primary division appears 

 to take place in the centrosome. If two are formed, there 



FIG. 32. Direct division of lymphocytes of a frog. (Arnold.) 



will be a simple nuclear spindle and the nuclear material 

 being equally divided and drawn toward them, two new 

 nuclei and two new cells will result; if there are four 

 divisions of the centrosome, there will be two nuclear 

 spindles and four new nuclei; if a greater number of divi- 

 sions of the centrosome, a still more complex arrange- 

 ment of spindles and a still greater number of nuclei will 

 be formed. In the sporozoa in which great numbers of 



