II. T. 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 can only be inoculated into rats, so is 

 strictly specific. (Cp. T. theileri). It is not the same, 

 therefore, as T. 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 T. 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'. 



