TRYPANOSOMES IN TSETSE-FLIES AND OTHER DIPTERA. 203 
Koch (20) as developmental stages of I’. gambiense were 
really ‘I’. grayi or a similar form. 
With regard to the origin of T. grayi, our first notion was, 
as I have stated, that it was identical with T. gambiense; 
when this notion was dispelled I was at first inclined to regard 
it, with Novy, as a parasite of the fly itself, and my discovery 
of the encystation seemed to confirm this idea, which, however, 
more mature reflections, especially considerations of the habits 
of the fly, made me give up. In my preliminary report (P.R.S., 
B 79, pp. 38, 39) I have stated my reasons for believing that 
T. grayi has a vertebrate host, and I have nothing to add to 
them. For various reasons we suspected T’. grayi to be an 
avian trypanosome, though we were not able to prove this, 
but Novy has shown that it is often impossible to find trypano- 
somes in bird’s blood microscopically when their presence can 
be demonstrated culturally. The fact that a fly (Oct. 10th, 
1905, p. 232), bred in captivity, becomes infected with T. 
egrayiafter feeding on fowls used to feed freshly caught flies, 
seems to me proof positive that T. grayi is anavian parasite ! 
but if so, I may point out as a corollary, it also suggests that 
T. grayi can be transmitted by the inoculative method. 
IV. Remarks on THE Lire-cycite AND Mops or T'RANSMISSION 
or 'T'RYPANOSOMES, 
The scientific study of the transmission of trypanosomes, 
and their relation to disease, dates from the publication of 
Bruce’s masterly reports on his investigations upon Nagana 
in Zululand in 1895 (8, 4). It is not necessary for me to 
dilate at length upon the results, well known to everyone, 
‘of these researches, admirable alike for their conception, 
execution and presentation, and marking, as Koch has well 
said, the beginning of one of the most important epochs in 
the study of the Protozoa. 
The publication of Bruce’s reports seems to have aroused 
almost immediately suspicions as to the true nature of sleep- 
1 Koch seems inclined to consider the crocodile as the vertebrate host of 
aes grayl or of other tsetse-fly trypanosomes, 
