Size in Relation to Body Surface 161 



that the oxygen tension at the center of such a body, 

 which is consuming oxygen uniformly throughout, 

 will fall to zero when the shortest radius of the ellip- 

 soid 



/5D 



xc 



A 



D being the coefficient of diffusion which was deter- 

 mined for frog tissues by Krogh ('19a), C the tension 

 of oxygen at the surface of the body, and A the rate of 

 oxygen consumption. The radius required turns out 

 to be 



■/ 



5 X 1.4 X 10- 5 X 60 X 0.209 



= 0.0148 cm = 148 micra, 



4 



when the culture fluid is in equilibrium with room air. 



When the tension is reduced to one-fourth that of 

 room air, the radius will be one-half as great, or 74 

 micra. The shortest radius of an ellipsoidal Colpoda, 

 whose volume is the mean of those individuals in figure 

 54 which were exposed to 40 mm. oxygen tension, is 

 40 micra; for the individuals exposed to room air it is 

 50 micra. There seems to have been no absence of 

 oxygen anywhere in the bodies, of either control or 

 low-oxygen individuals. 



Oxygen consumption. It is worth while to find out 

 whether there are any other unicellular species where 

 oxygen will barely penetrate to the center. This can 

 be judged for those species whose rates of oxygen con- 

 sumption have been measured. In table 13 the avail- 

 able determinations have been listed. 1 Among the cili- 

 ates, which are the only species whose body size is suffi- 

 cient to bring them into consideration, none has a 

 greater rate of oxygen consumption than Colpoda has, 



1 Numerous other determinations of oxygen consumption have been 

 referred to the dry weight of the organisms used, as by Callow ('24), 

 Meyerhof ('25), and Copping ('29). Still other determinations have not 

 included any measure of the quantity of organisms used. 



