1910.] 



Trypanosoma lewisi hy the Rat-Jiea. 



275 



question whether or not a sexual process takes place in the course of the 

 development. 



Experiments A and B were arranged so as to eliminate possible infection 

 other than by fleas, and to separate " direct " from " cyclical " infection, so 

 that if infections did take place, it would be clear whether they were the 

 result of the direct method, the cyclical method, or both. When it had been 

 ascertained in the course of Experiments A and B that infections did take 

 place, but not by the direct method, these experiments were prolonged in 

 order to determine if fleas, once infective, retain the infection so as to infect 

 a succession of healthy, clean rats. This is also a clear issue in Experi- 

 ments C and D, which were arranged with a view to determine further by 

 -direct observation, and within narrow limits, (1) the length of the incubation- 

 period in the flea, that is to say, the length of time required for the parasite 

 ■to establish itself in the flea and render it infective, and (2) the length of the 

 multiplication-period in the rat, that is to say, the length of time from the 

 actual inoculation of the parasite into the rat until the trypanosomes cease to 

 multiply in the rat's blood. As is well known, when a rat is first infected 

 with T. leivisi, the parasite multiplies in the blood very rapidly. After a 

 certain length of time the multiplication ceases entirely and rather suddenly. 

 When rats are infected artificially in the laboratory by the ordinary method 

 of intra-peritoneal inoculation of blood from an infected rat, the 

 multiplication-period lasts for some 10 or 12 days after inoculation, as a 

 rule ; but it by no means follows that multiplication lasts for the same 

 length of time when the inoculation is performed by fleas. If the length of 

 the multiplication-period is known, and is found to be a constant, it is evident 

 that in any given experiment the time when the infection of the rat took 

 place can be ascertained by simply observing when multiplication of the 

 trypanosome ceases in the blood and then deducting the known period; and 

 further, if the infection is a first one and the time during which the fleas 

 were exposed to infection was short and is known, then the length of the 

 incubation-period in the flea can be calculated also. Experiments D and E 

 were undertaken with the special object of determining the incubation-period 

 in the flea. 



II. Account of the Experiments. 

 (1) General. — The same arrangements were made for all the experiments, 

 which differ only in details. The fleas used in all cases had been bred in 

 flea-proof cages in the laboratory, the parents being some 50 fleas which were 

 obtained in the early autumn of 1908 from rats trapped in the neighbourhood 

 of the Sutton Broad Laboratory in Norfolk. These fleas multiplied in our 

 cages during last winter and spring to produce literally thousands ; we have 



