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Trypanosoma lewisi 
forms which are reinoculated into the rat -when an infected flea is 
feeding on it, and so produce infection. We tried to find this or any 
other stages of development in the salivary glands or probosces of 
heavily infected fleas, but never succeeded in finding anything. 
Synopsis of the life-cycle of T. leivisi in the gut of Ceratophyllus fasciatus. 
Apart from these considerations, the fact remains that T. lewisi 
undergoes a well-marked development with a definite conclusion in 
the gut of Ceratophyllus fasciatus, a development which cannot be 
merely considered as a natural culture, because apart from forms 
found also in cultures, stages of development were observed (small 
trypanosomes) which were never found in culture. 
4. Development of T. lewisi in the rat louse 
(Haematopinus spinulosus ). 
The work on the development of T. lewisi in the invertebrate host 
has been done merely in connection with the rat louse; the papers 
referring to this subject have been quoted in the beginning of this 
article. 
The development in the louse is a very irregular one and is not to 
be compared with that which takes place in the flea. As we have seen, 
the rate of infection in fleas fed only once on an infected rat is 36'9°/o- 
If fleas are taken haphazard from an infected rat almost every specimen 
shows developmental forms. This is not the case with the louse. 
During the summer of 1909, 270 lice were dissected in Cambridge, 
but only normal and degenerated forms were to be found in the gut. 
In the early spring of 1910, 50 lice were dissected in Amsterdam with 
