142 
GENETICS: W. H. TALIAFERRO 
Proc. N. A. S. 
which to work. It affectively eliminates growth factors from our compu- 
tations. 
The most interesting feature in the study of the variability exhibited 
by the "pure line" is that once the organism has reached the adult stage 
of infection the coefficients of variation are extremely low. Table 1 
shows the constants for rat 105. The coefficients of variation for total 
length and parabasal-nucleus are 2.80% and 2.21%, respectively. It 
is to be noted also that these two characters show a lower coefficient of 
variation than any of the others. After considering this subject in great 
detail we have come to the conclusion that these two characters give the 
truest index of the variability in the "pure line." While there is not 
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 
Daaj of Blood Infection 
FIG. 2 
Graph showing the changes in the mean and coefficient of variation for total length 
in rats 116 and 105. The infection in 116 was started from a single organism 
and 105 was inoculated from 116. The trypanosomes disappeared from 
the blood of rat 105 on the 32nd day and from 116 on the 72nd day. 
much doubt that posterior-parabasal is more variable than either of 
these two, we feel that much of its variability as well as that of the other 
distances with high coefficients is due to the difficulty of making the 
measurements. 
Conclusion. — We can draw the following conclusions from this part 
of the work: Trypanosoma lewisi reaches an "adult" stage in its develop- 
ment in the rat in about 25 days after it appears in the blood. Once this 
stage is reached there is practically no division or growth. Due to the 
elimination of growth factors, the organisms show a very low coefficient 
of variation in "pure line" infections, provided they are measured after 
the "adult" stage is reached. These facts make T. lewisi a very favorable 
organism in which to study size and variation. 
