162 Regulation of Size in Unicellular Organisms 



and the shortest radius of none of these forms is 

 greater. Complete lack of oxygen apparently does not 

 occur anywhere in the bodies of individuals in culture 

 media which are in equilibrium with air. 



The lowered tension of oxygen, in the experiment of 

 figure 54, therefore exerted its effect not by denying 

 oxygen to the tissue. It may have acted by lowering 

 the concentration of oxygen in the tissue in connection 

 with some process which was thus limited. One pos- 

 sible way in which a low tension might be conceived 

 to have a significant influence is through its liberation 

 of the processes which form acid in tissues, such as was 

 investigated, particularly by Meyerhof ('25) and 

 termed by him the Pasteur effect. 2 The effect is pres- 

 ent in yeast and probably a great many other species 

 and tissues. Shoup ('29) found that a very considerable 

 tension of oxygen was required at the surfaces of certain 

 bacteria in order that their oxygen consumption might 

 remain maximal. From this it appears that in the 

 Colpoda the tension of oxygen was certainly low enough 

 to make its shortage felt, even though oxygen was not 

 actually absent in any part of the body. 



But, oxygen is not necessary for survival, as was 

 shown by Spallanzani (1803), and by Pasteur ('61) to 

 be true for many kinds of unicellular organisms; this 

 has since been widely investigated. Although Para- 

 mecium is killed in a few minutes by lack of oxygen 

 when suspended in a drop of medium (Zoethout, '99; 

 Putter, '05), it and other ciliate species survive for 

 many hours or days when put into large volumes of 

 fluid without oxygen (Putter, '05). 



Whether oxygen penetrates to the interior of an 

 individual by diffusion alone can hardly be strictly as- 

 sumed. It is conveyed by water in those species which 

 have unidirectional migrations of water, certainly in 



a Pasteur (76). 



