204 BIOLOGY OF THE PROTOZOA 



but a renewed or purified organization such as the individual received 

 when it was formed. With the processes of division the old differ- 

 entiations are lost by absorption, the organization is de-differen- 

 tiated and the protoplasm has a renewed potential of vitality. 



In order to understand the relations of division to the chain of 

 metabolic activities we should know more about the conditions under 

 which division occurs, and the "causes" of division. There is very 

 little real evidence for conclusions in this matter but there have been 

 many theories. The latter for the most part are based either upon 

 analogies with physical phenomena or upon hypothetical "spheres 

 of influence" of morphological elements of the cell. They have been 

 developed in the main to interpret phenomena of division in meta- 

 zoan cells, particularly in egg cells, and fall completely to the ground 

 when applied to division of Protozoa. So it is with the contractility 

 hypothesis of Heidenhain, Driiner and others who see in the spindle 

 fibers and astral rays a contractile system whereby the nucleus and 

 cell are divided in a strictly mechanical manner. The intra- 

 nuclear spindle and the absence of cytoplasmic rays in the great 

 majority of Protozoa are enough to show that such physical inter- 

 pretations do not reach to the root of the matter. The "spheres of 

 influence" hypotheses, based upon the kinetic center of the cell and 

 its influence on the cytoplasm, was developed by Boveri in the 

 attempt to associate cell growth and the causes of division. The 

 "energid" theory of Sachs and Strasburger was an analogous effort 

 to trace the causes of cell division to increasing volume of the cell 

 through growth, each nucleus having its sphere of influence in the 

 cj-toplasm and dividing when the volume of the cell outgrows the 

 sphere of activity of the nucleus. The Kernplasmverhdltnis theory 

 of Hertwig was based upon somewhat similar grounds. Accord- 

 ing to this the volume of the nucleus bears a certain normal relation 

 or ratio to the volume of the cytoplasm in young actively func- 

 tioning cells, evidence of which in Frontonia was given by Popoff 

 (1909) and by Hegner (1920) in the equidistant distribution of nuclei 

 in various species of Arcella. With increasing age this ratio is 

 altered to the advantage of the cytoplasm until division of the cell 

 restores the normal ratio. With uninucleate forms such as Para- 

 mecium or Frontonia there is some evidence of change in relative 

 volumes, and careful measurements by Popoft' (1909) and other 

 followers of Hertwig are adduced to support the hypothesis. In 

 these forms the volume of the nucleus is proportionally reduced 

 until just prior to division when the nucleus rapidly increases in 

 volume and divides. In Vroleptus, Vronychia and similar forms, 

 however, the many nuclei fuse to form one compact and relatively 

 small nucleus prior to division. It would seem that such changes 

 in relative volume of nucleus and cytoplasm are better interpreted 

 as the effects of underlying conditions which cause division rather 

 than as the cause of division themselves. 



