86 Regulation of Size in Unicellular Organisms 



reproduction of individuals. From this it could be 

 hypothecated either that a foraminiferan is analogous 

 to a multinucleate heliozoan, or that it is analogous 

 to a colonial flagellate whose individuals have not be- 

 come separated. 



Nuclear division. Finally, it is worth-while noting 

 that nuclear fissions in protozoa may be regulated en- 

 tirely separately from cytoplasmic fissions. This is 

 the case in all multinucleate species which have no defi- 

 nite number of nuclei per individual. And it is signifi- 

 cant that in many such species all the nuclei divide at 

 one time. This was noted by Pfitzner ('86) in Opalina, 

 and by Gruber ('87) in Holosticha, and by many 

 others. In Actinosphaerium, individuals may exhibit 

 an arrangement of the various phases of mitosis in 

 zones, showing that, for instance, the prophases 

 started in a few nuclei of one region and then spread 

 progressively throughout the protoplasm until, ap- 

 parently, all the nuclei were induced to divide in se- 

 quence. This is an extraordinary degree of coordination. 



Summary. Instances have now been cited in which 

 cytoplasmic fission, nuclear fission, growth in bulk, 

 and aging in time stood in all the possible relationships 

 of dependence and independence. In any particular 

 instance these four functions may or may not be con- 

 catenated. And those combinations out of the four 

 that normally coexist may become dissociated at least 

 in part. The regulation of size is not always bound up 

 with any one of the other three. Where it is so bound, 

 a tangible factor is present, capable of setting the limit 

 to size. Where none of these factors has to do with 

 size, it is possible that other internal mechanisms are 

 present to control size, without their having actually 

 been influenced by size itself. If no internal factors 

 limit growth in the time before reproduction, then size 

 as such remains as its own determiner. 



