Vol. 7, 1921 
GENETICS: W. H. TALIAFERRO 
139 
been described by Jennings 3 and others in free-living protozoa which 
differ among themselves but are per se constant in size? As will be seen 
later, these questions cannot be approached with any degree of exactness 
until a thorough study is made of the changes in mean size and varia- 
bility throughout the course of an infection. The present paper deals 
with these changes in size and variability and with the first of the ques- 
tions enumerated above. The other three questions are to be taken up 
in a later report. 
A study of inheritance in a parasitic protozoon such as T. lewisi is of 
interest from several points of view. In the first place the results are 
of interest from a comparative standpoint when considered in the light 
of recent advances in our knowledge of the genetics of free-living species. 
In the second place, the results may be of value in the interpretation of 
the results of the many studies on the production of strains of parasitic 
organisms which exhibit new characteristics. Finally the work is the 
first of a program of investigations, the ultimate object of which is a study 
of the mechanism of the formation of new lines exhibiting such characters 
as arsenic-fastness and the inheritance of these characters after passage 
through both the vertebrate and the invertebrate hosts. 
While lack of space prevents a discussion of technique in detail, it 
may be noted that every precaution was taken to use microscopical tech- 
nique such that the trypanosomes would be free from shrinkage and dis- 
tortion. All measurements were made from camera lucida outlines 
drawn at a magnification of X3000. The unit used in measuring the 
drawings was 3 mm.; consequently all of the determinations given in 
this paper are in actual microns. In making the determinations 100 
specimens, taken at random without selection, were drawn in each case. 
In isolating single organisms with which to start "pure line" infections, 
a sensitive mercury pipette was used in conjunction with a Barber pipette 
holder. Figure 1 is a diagram of a trypanosome indicating the various 
parts of the organism and the distances measured in this work. The 
names of the parts of the trypanosome run vertically and the abbrevi- 
ations of the six distances run horizontally. 
Size and Variation throughout a " pure line" Infection. — The infection 
in the rat can be divided into three periods: (1) the incubation period 
lasts from 1-7 days and is the time which elapses between inoculation 
and the first appearance of the trypanosomes in the blood. (2) The 
multiplication period starts with the first appearance of the trypanosomes 
in the blood and lasts for 10-25 days. This period is characterized by 
the great variations in size due to the growth and the multiplication of the 
trypanosomes. (3) The period of "adult" infection follows the second 
period and is characterized by the absence of all growth and multi- 
plication. This period lasts from one to many weeks at the end of 
which time the trypanosomes disappear from the blood and the rat is 
