REPRODUCTION 205 



the organization being de-differentiated, 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 metazoan 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 intranuclear spindle and the absence of cytoplasmic rays in 

 the great majority of Protozoa are enough to show that such physical 

 interpretations 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 

 cytoplasm and dividing when the volume of the cell outgrows the 

 sphere of activity of the nucleus. The Kernplasmverhaltnis 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 Fronionia 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 Fronionia there is some evidence of change in relative 

 volumes, and careful measurements by Popoff (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. Looper (1928) more recently, by mech- 

 anical stimulation, caused Aciinophrys sol to fuse with enucleated 

 fragments from other individuals. This led to change in the nucleus- 

 cytoplasm ratio to the advantage of the cytoplasm. Such forms 

 divided from one-half to two times faster than the controls. If, 

 on the other hand, some cytoplasm is cut away, the reduced cells 

 (100 cases) divided on the average in eighty-eight hours, while 



