Regulation of Size in Unicellular Organisms 



reproduction ceased after some days of infection, and 

 a maximum growth was then attained by each para- 

 site. The lengths of the individuals in this sample are 

 so similar, that three-fourths of the individuals differ 

 from the mean length by only 2.5 per cent of the mean 



Z5 



^20 



p- 



Z3 

 to 



S 15 



E 



i5 to 



E 



3 











10 20 30 



Total body length in micra 



Figure 1. Distribution of total body lengths in 100 individuals of Trypan- 

 soma lewisi. The standard deviation is 2.68 per cent of the mean length, 

 which is the lowest coefficient of variation reported for any measurement 

 on any unicellular organisms. The trypanosomes were a clone in the blood 

 of a single rat on the 49th day of the infection. (Redrawn from Taliaferro, 

 '23). 



length. This value for the tendency to deviate from 

 the mean is the coefficient of variation. The uniform- 

 ity among this group of a hundred parasites of one 

 race is much greater than could be obtained among a 

 hundred sticks if an attempt were made to cut them to 

 a uniform length without any means of measurement. 

 It is much greater than the uniformity of the heights 

 of the men or of the women in any human population. 



