Inheritance of Size 61 



may be supposed, the universality with which uniform- 

 ity of size is taken for granted. 



Trypanosoma. Among flagellates the inheritance 

 of size has been investigated with particular results in 

 the parasitic species Trypanosoma lewisi living in rats. 

 Some of the evidences for the exact inheritance of size 

 in this species were reviewed by Hegner ('21). Talia- 

 ferro ('23) proceeded to infect rats with single individ- 

 uals of the organism, giving rise to a pure line which 

 was transferred to many rats. It was found that as 

 the infection increased in age the mean body length 

 of the trypanosomes gradually increased (figure 26). 

 After some days the parasites attained sizes which 

 were not only maximal but also excessively similar 

 (figure 1). The coefficient of variability after the 

 twenty-fifth day of infection never exceeded 4.7, and 

 was as low as 2.2. The low variability was taken to 

 indicate that reproduction had ceased, while all of the 

 organisms present were able to grow until they were 

 adult in size. This interpretation was proved by 

 counting the organisms which were undergoing fis- 

 sion, and by counting the concentrations of individuals 

 in the host's blood (Taliaferro '24). Undoubtedly the 

 low variability resulted also from the great constancy 

 of the environment furnished by the host. 



Several rats were infected with the same pure line of 

 trypanosomes, and it was found that after the initial 

 period of growth the identical mean length and mean 

 width were attained by the parasites in each of the rats 

 (figure 26). After the twenty-fifth day of infection 

 the mean lengths of four infections were between 29.3 

 and 32.1 micra, and the mean widths, which always had 

 higher coefficients of variability, were between 1.42 

 and 1.74 micra (fourteen determinations). 



Actinophrys. In the naked heliozoan Actinophrys 

 there exists one demonstration of size inheritance. Be- 



