Vol. 7, 1921 
GENETICS: W. H. TALIAFERRO 
165 
parasites the natural insect vectors are the rat fleas, Xenopsylla cheopis 
and Ceratophyllus fasciatus. In working out the life history of certain 
species of trypanosomes some of the earlier investigators described what 
they considered to be sexual phases in the life history of the parasite. 
Practically all of these observations are open to entirely different inter- 
pretations, so that at the present time, although a number of proto- 
zoologists feel that there is a sexual phase in the life cycle, no such phase 
has been demonstrated. This is notably true in the classic researches 
of Minchin and Thomson 4 on the life history of T. lewisi in the rat flea. 
Although they looked carefully for conjugation they found no evidence 
of it. 
While there is no evidence of sexual phenomena in the trypanosomes 
during passage through the invertebrate host, there is evidence that such 
passage exerts a profound effect on such things as acquired physiological 
characteristics. Gonder 5 for example found that arsenic fastness was 
transmitted from rat to rat but was lost by passage through the louse. 
Miss Robertson 6 found that strains of T. gambiense showed marked 
changes in their characteristics after passage through the tsetse-fly. 
This led this author to say, "It seems clear that the cycle in the fly as a 
whole, whether conjugation occurs or not, has much of the biological 
significance of the process." 
As we have seen passage of the same "pure line" through a number 
of rats does not increase the variability of the "pure line." Passage 
through the flea, on the other hand, invariably increases the variability. 
These experiments were carried out in the following manner. Fleas 
which were carefully raised in the laboratory and which were known to be 
free from infection were allowed to bite animals infected with a given 
"pure line." After about a week (during which time the trypanosomes 
were undergoing their development in the flea) other rats were infected 
either by teasing up a single flea and injecting it intraperitoneally or by 
allowing the rat to lick up the moist feces, which is the natural mode of 
infection. The trypanosomes in the rat which was infected from the 
flea were measured on the 30th day of the blood infection. Six such ex- 
periments have been successfully carried out. The following is a fair 
example of all of them. The pure line in rat 105 showed a coefficient 
of variation of 2.80=*=. 13% for total length. After passage through a 
single specimen of X. cheopis it was measured in rat 163. Here the 
coefficient of variation increased to 5.24 ± .25%, a difference of 2.44 =±= .28%. 
As we have seen above, no such increase in variability has been observed 
during passage of the "pure line" from rat to rat. From these experi- 
ments we must conclude that the "pure line" breaks up during passage 
through the invertebrate host. This is analogous to the results of Jenn- 
ings 7 in Paramecium after conjugation and in a recent publication Miss 
