CELL DIVISION 



105 



to a certain size, appears to separate into two or more 

 equal parts, each of which takes on an independent 

 existence. The structure of certain cells, as the bacteria, 

 is still controversial. Some believe them to have large 

 nuclei and see in them appearances analogous to those 

 of karyo kinesis at the time of division. It is usual, 

 however, to describe the division of these organisms 

 as taking place by direct division, i.e., simple fission 

 without karyokinesis. 



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



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 



