388 



TR YPA NO SO MI DM 



the cyst which is formed by the periplast of the original trypanosome, 

 and enter red blood-cells, in which they develop into sexually differ- 

 entiated trypanosomeS' — i.e., females with one and males with two 

 nuclei. These are the forms which infect iYie Lamus, or invertebrate 

 host, and do not multiply in the vertebrate host. 



Method of Transmission. — The blood of the infected vertebrate is 

 not always infective for the invertebrate host . Thus Miss Robertson 

 has shown, with regard to T. castellanii, that the tsetse-fly cannot 

 be infected by feeding just before an outburst of multiplication in 

 the vertebrate host, or during the period of destruction which 

 precedes a paucity period, or at the summit of an exalted period, 

 or during certain periods of rapid multiplication, when the absolute 

 and relative numbers of the short forms mentioned above are 

 diminished. 



Immediately after infection the invertebrate host can mechani- 

 cally convey the infection to a clean host, and this power persists 

 for about twenty-four hours, after which the invertebrate host 

 becomes non-infective, and remains so for a varying period, which 

 was found by Kleine not to be less than eighteen days as regards 

 Glossina palpalis and T. castellanii, and by Kinghorn and Yorke 

 to be about fourteen days in G. morsitans infected with T. rhodesiense, 

 after which the flies become again infective, and apparently remain 

 so for a very long period, probably for the rest of their lives. These 

 facts prove definitely that the parasites undergo some changes of an 

 important nature in the flies in question, and that the second in- 

 fective period cannot possibly be classed as mechanical. The fact 

 that Lamus megistus remains infective ten to twenty-five days after 

 feeding on a host infected with 5. crvizi also proves that the infection 

 is not mechanical. We therefore draw the conclusion that the 

 trypanosome undergoes part of its life-cycle in the invertebrate host, 

 and the first question which naturally arises is the fate of the so-called 

 ' male ' and ' female ' forms found in the blood of the vertebrate. 

 Are they true male and female forms, and do they conjugate and 

 form an ookinete or zygote or do they not ? 



Conjugation.— It is difficult to be certain that conjugation has 

 been seen, and not division. It has been described by Keysselitz 

 as occurring in leeches fed on a carp infected with Trypanoplasma 

 borreli, by Prowazek in lice fed on rats infected with T. lewisi, 

 and by Flu in Crithidia melophagia, to which reference will be made 

 under the heading of Crithidia. 



On the other hand, careful observers like Miss Robertson and 

 Captain Patton have quite failed to see this process in their studies 

 of trypanosomes and herpetomonads. The conclusion is that 

 conjugation has not been proved to be present in trypanosomes 

 so far. 



Development without Conjugation. - According to most observers, 

 there is no conjugation, but development takes place asexually in 

 the invertebrate host. This development varies in different species, 

 and may be classified as follows : — • 



