274 Messrs. Miuchin and Thomson. Transmission of [Jan. 7, 



has shown that a third method (c) is possible, partly mechanical and partly 

 biological, resulting from the late multiplication of the parasites in the 

 intestine and their subsequent introduction into the body of the bitten 

 animal." It may be pointed out, first that the mechanical element is present 

 equally in {a), (b), and (c), and secondly that the question of " the conjugation 

 of two individuals " seems to us of quite secondary importance in this 

 connection. There are non-sexual cycles of development as well as sexual ; 

 in the malarial parasite, for instance, there is a non-sexual cycle, which takes 

 place in the human body, and a sexual cycle, which takes place in the 

 mosquito. At the present time the belief that a sexual cycle in the develop- 

 ment of trypanosomes takes place in the invertebrate host is largely an 

 assumption, based on the analogy of the malarial parasite, and in need of 

 objective proof. 



We desire to approach the subject in an unbiased manner, and we would 

 recognise, for the present at least, only two methods of transmission, of 

 fundamental scientific and practical importance, which we shall term the 

 ^' direct " and " cyclical " methods respectively. In the direct method the 

 invertebrate acts merely as a suitable instrument in the transmission. 

 Experimentally the chances that the invertebrate will convey the infection to 

 a susceptible animal by the direct method are greatest immediately after it 

 has contaminated its proboscis by feeding on an infected one, and these 

 chances gradually diminish and cease altogether within a comparatively 

 limited period of time. And experimentally the invertebrate is found to have 

 exhausted its power to infect in the process of cleaning its proboscis at the 

 feed which follows next after that on the infected animal. In the cyclical 

 method, on the other hand, the invertebrate is more than a suitable instru- 

 ment in the transmission : it acts as a host in whicli the parasite establishes 

 itself and maintains the existence of its species. Infection l)y tlie cyclical 

 method can take place only after the parasite has establislied itself, and it 

 can then continue to take place so long as the parasite maintains its existence 

 in the invertebrate. Experimentally it is found that an invertebrate of the 

 right species, after having fed on an inlected animal, may feed many times on 

 susceptible animals without conveying the infection, and may then, without 

 having fed again on an infected animal, become infective ; and, further, that 

 once infective, it may remain infective for an indefinite period of tinu^ — 

 poHsil)ly for the rest of its life. Of such an invei'tebrati; we may say tliat it 

 is a true host, and tiiat the parasite it transmits passes through a cycle of 

 development witliin it, nu;aning by tlio word cycle a series of changes and 

 generations which follow one another in more or less ([('finite order in the 

 develo|iin('nt and iinilti|)li(;ati()n of tli(^ pai'asite, and heaving it an ()p(ui 



