8 Regulation of Size in Unicellular Organisms 



Food vacuoles. But the visible "inclusions" of a 

 protoplasm are as variable as its water content. The 

 combined volumes of all the food vacuoles have been 

 determined by Unger ('26) in a large number of in- 

 dividual Paramecia. The modal volume of the vacu- 

 oles in a race of Paramecium aurelia turns out to be 

 one-sixth of the total body volume of modal individuals 

 of the same race as measured by Erdmann ('20). A 

 few estimates of food vacuole volumes made by Eisen- 

 berg ('26) in Paramecium caudatum indicated that 

 the vacuoles occupy 7 per cent of the body volume. A 

 Paramecium carrying on active digestion, therefore, 

 has reason to be larger than a starved one by at least 

 this much. 



J. A. Dawson ('19) showed that Oxytrichse may in- 

 gest their cousins of the same species until they be- 

 come truly giant individuals. In one instance fifteen 

 victims had been ingested, and the giant had thus 

 made itself at least eight times as large as its non-can- 

 nibalistic relatives. Most of the extra size consisted 

 at first of unincorporated food, and could in no wise be 

 considered as part of the body substance of the giant. 

 The giant was large not in its own right but because 

 of foreign puffing. Similar instances were described 

 by Mast ('09) ; a Didinium may ingest a Paramecium 

 of ten times its own volume, and then pass through 

 three or four generations on the strength of the food 

 and the bulk thus obtained. 



Contractile vacuoles. Contractile or excretory 

 vacuoles do not ordinarily constitute so large a portion 

 of the body mass as do food vacuoles. In an average 

 Ameba the contractile vacuole was one-hundredth of 

 the body volume, and in one extreme case was as much 

 as one-twentieth (Adolph, '26). 



For the one substance water, calculations can be 

 made as to how fast the material is turned over or re- 



