The Rate of Multiplication 79 



but in hardly any instances were changes of adult size 

 looked for. The "mitogenetic radiations" from onion- 

 root tips were said by Baron ('26) to increase the num- 

 bers of buds formed by yeast cells, but by Urbanowicz 

 ('27) not to change the rate of multiplication in Para- 

 mecium. 



Every factor which significantly modifies fission 

 rate requires study in relation to adult size. If size is 

 affected without the fission rate, then the influence is 

 upon the rate of growth; if fission is affected, then 

 growth may or may not be also affected. 



Retarded fissioning. The hastening of fission under 

 circumstances that are already adequate for the con- 

 tinued growth and multiplication of individuals is, 

 therefore, of rare occurrence and experimentally diffi- 

 cult to induce. But the inhibition of reproduction un- 

 der circumstances where growth continues, occurs fre- 

 quently, and in response to definite stimuli. 



In numerous cases double organisms, or organisms 

 with excessive body substance, have been observed by 

 chance. Such modified forms have been found to per- 

 sist for many generations in Paramecium (Simpson, 

 '01 ; Jennings, '08a; Stocking, '15), in Didinium (Mast, 

 '09), and in Oxytricha (Dawson, '20). In most in- 

 stances it is impossible to distinguish between mere 

 abnormality of size and true failure of formed progeny 

 to separate. Only in the extreme cases can the indi- 

 vidual be defined; in the borderline cases no criterion 

 has yet been discovered which will differentiate be- 

 tween single and multiple individuals, if in reality there 

 is any objective differentiation. 



Incomplete fissioning. In relatively few instances 

 have methods been found by which incomplete fission- 

 ing can be induced. 



After cutting. Balbiani ('91, '92, '93) found that in- 

 jury by cutting might result in the formation of mon- 



