94 Regulation of Size in Unicellular Organisms 



seems to furnish a tangible determiner of adult size. 

 This growth cycle is still more evident in the many 

 species where more than two individuals, usually four, 

 are reproduced at one time. 



From this point of view, fission constitutes a "re- 

 juvenescence"; it enables growth, which came almost 

 to a standstill before fission, to be resumed, and to be 

 resumed most rapidly at once. Thereafter the growth 

 intensity gradually diminishes until the next fission. 

 It might be assumed that assimilation goes on at a 

 fairly uniform rate, but that it is slowly caught up with 

 by dissimilation; for it is seen in some of the data upon 

 rate of growth that a level of maximum body size is 

 reached shortly before fission. 



Fission is undoubtedly a major factor in growth in 

 some species. In other species, such as the filamen- 

 tous bacteria, where growth is in one dimension only, 

 fission has less effect upon growth (see figure 16), 

 and in a few species the effect is not measurable. 



On the other hand, reproduction is never regulated 

 wholly by growth. Growth has been shown to be one 

 prerequisite, yet not always indispensable; for age or 

 other factors may become potent enough, at least in 

 some species, to overcome its absence. There is evi- 

 dence, now to be presented, that special factors have 

 different effects upon reproduction and upon increase 

 in mass. 



3. Regenerating Capacity and Size 



Thus far the rate of regeneration has been consid- 

 ered in relation to the onset of reproduction. But size 

 factors are also concerned in the ability of cut indi- 

 viduals to survive and regenerate. 



Criterion. The criterion of regenerating capacity 

 among unicellular organisms has often been merely to 

 observe whether they were able to reproduce. But in 



