0. Basile 373 
Wenyon in Pulex irritans , while in Leishmania, as has been already stated, 
the posterior end is conical. 
It can be deduced from these indications that an accurate study of the 
complete developmental cycle of the various Herpetomonas and Crithidia al¬ 
ready described in fleas reveals certain morphological differences between 
these and the developmental forms of Leishmania in the same insects; on the 
other hand, the morphological study, if it be not supplemented by biological 
and experimental data, may be sometimes insufficient. 
In this respect I repeat to-day what I have written ever since I began 
these studies in 1911, “given the constant presence of Protozoa, often morpho¬ 
logically similar, in the intestine of insects, anyone who studies their develop¬ 
mental cycle rather than their morphology is able, by means of experiments, 
to infer whether the said Protozoa are etiological agents of disease.” 
It is, in fact, necessary to bear in mind that the herpetomonad and crithidial 
forms found in the intestine of fleas are not always Insectan Flagellates, since 
developmental stages of trypanosomes which possess a herpetomonad or cri¬ 
thidial phase also occur in fleas. Minchin and Thomson found in the rat flea 
(Ceratophyllus fasciatus) the developmental stages of Trypanosoma lewisi ; this 
parasite, in some of its forms, assumes all the characteristics of Herpetomonas ; 
similar phases of T. lewisi have also been discovered in Ctenocephalus serrati- 
ceps (Noeller), in Pulex irritans (Wenyon), and in Xenopsylla cheopis, experi¬ 
mentally infected; we cannot, therefore, exclude the possibility that they 
may be found in fleas infected in nature. 
In reference to the relations between pathogenic forms of Herpetomonas 
and Trypanosomes the researches dealing with Schizotrypanum cruzi are 
interesting; in the digestive tract of Triatoma megistus taken from the houses 
of Minas Geraes, Brazil, Chagas observed Protozoa of the herpetomonad type; 
monkeys inoculated with the intestinal contents of these Hemiptera became 
infected with Trypanosomes; this discovery led Chagas to ascertain the exist¬ 
ence of a trypanosomiasis previously unknown, which from its clinical symp¬ 
tomatology has been called “parasitic thyroiditis”; the parasite, on account 
of certain characteristics of its developmental cycle, has been called Schizo¬ 
trypanum cruzi. 
Results identical with those of Chagas have been obtained by Lafont in 
the island of Mauritius, and by Carini at San Paulo. Recently, also, Tejera 
when examining the intestinal contents of Rhodnius prolixus in Venezuela 
found these Hemiptera infected with flagellates which, when inoculated into 
animals, produced a trypanosome-infection identical with that caused by 
Schizotrypanum cruzi. These interesting investigations of Chagas into trypano¬ 
somiasis, or parasitic thyroiditis, indicate the importance which the researches 
of experimental protozoology upon the various Herpetomonas and Crithidia 
found naturally in fleas have in relation to leishmaniasis. 
Since 1910 I have been seeking to solve the important problem of the 
pathogenic significance of the Protozoa of fleas taken from children and dogs 
