180 Regulation of Size in Unicellular Organisms 



conditions it was reported by Hartmann ('28) that, 

 after the artificial removal of cytoplasm in Ameba and 

 during subsequent regeneration, the size of the nucleus 

 remained unchanged. 



Excision of nuclei. Variations of the number of nu- 

 clei present in Arcella were produced by Hegner ('20) 

 by cutting away some nuclei. As already shown in 

 figures 28 and 29, the body volume was, within a few 

 generations, regulated in approximate proportion to 

 the number of nuclei present. But very frequently 

 the number of nuclei was restored to its original value 

 by a process of fission which consisted of nuclear di- 

 vision and empty shell formation without cytoplasm 

 separation. This sort of adjustment is not known 

 to be connected in any simple manner with body size, 

 and offers a further problem in regulation. 



Pieces of Stentor, in which both the nucleus and the 

 cytoplasm had been diminished by cutting, always 

 gave rise to progeny of the body size normal to the 

 race (Burnside, '29). The size of the nucleus, as well 

 as the number of nuclei and the volume of the cyto- 

 plasm, was apparently regulated at its old value by 

 any unit of the body that was capable of carrying on at 

 all. Hence it is probable that the regulator is not itself 

 any structure of definite size. 



Summary. It may be concluded that within a multi- 

 nuclear clone the number of similar nuclei and the vol- 

 ume of cytoplasm are usually proportional. This im- 

 plies nothing about the mechanisms of control of the 

 size of the whole body, for it is also found that under 

 constant conditions the size is preserved so that each 

 individual has about so many nuclei. Some observa- 

 tions given in table 5 upon Actinosphaerium cultivated 

 under highly constant conditions, show that the varia- 

 bility of body volume is hardly any greater than in the 

 uninuclear Actinophrys; yet each Actinosphaerium 



