35i 
n. E. lewisi. —In a certain percentage of rats all 
over the world. Trypanosomes may not infrequently 
be cultivated from the blood of rats in which repeated 
blood examination has been negative (Novy). 
Symptoms. —Non-pathogenic, with very rare ex¬ 
ceptions. It pan only be inoculated into rats, so is 
strictly specific. (Cp. T. theileri). It is not the same, 
therefore, as E. rabinowitschi v. criceti of the hamster. 
Blood Examination. —Rats naturally infected may 
shew few or very numerous trypanosomes : they may 
remain infected for months. In inoculated rats, 
rosettes may be found during the first few days. 
Sub-inoculation. —The best method is intraperi- 
toneally, but sub-cutaneous injection suffices. Piebald 
or old rats may be refractory. 
Immunity. —Rats that have become free from 
trypanosomes are immune. By injecting such rats on 
several occasions with E. lewisi , their serum acquires 
protective properties. 
Morphology. —7-30/^ by 1 ‘5-3/^. The posterior end 
is drawn out into a point. The blepharoplast is rod¬ 
like and transverse to the long axis. The nucleus 
is oval and situated at the junction of middle and 
anterior third of the body. The protoplasm shews 
fine chromatin granules, but not coarse ones as the 
pathogenic trypanosomes do. Division, as in all 
trypanosomes, is longitudinal, but owing to continued 
division of the nuclei and delayed division of the 
protoplasm, rosettes are also formed. These consist 
of a number of minute trypanosomes with their 
flagella situated peripherally (Fig. 109). They are 
characterized by the fact that the blepharoplast has 
moved close to the nucleus or even anterior to it. 
The blepharoplast resumes its ordinary position when 
the rosettes break up. 
