70 
Insect Flagellates , etc. 
find any traces of hereditary infection in the sheep-ked. In several cases I 
saw structures which at first glance could be mistaken for flagella of the 
parasite, which, according to Porter, “are found floating freely in the vitellus 
of eggs.” A careful examination of these structures, however, convinced me 
that they were spermatozoa, which find their way into the ovaries from the 
receptaculum seminis. It is a noteworthy fact that, although the authors 
named believed that hereditary transmission exists, they have all failed to find 
any traces of the flagellates thus inherited either in the larvae or in newly 
hatched “keds."’ Only after prolonged feeding on sheep’s blood do the 
flagellates make their appearance in the gut of the young “keds." The natural 
deduction from such a fact would be the assumption of a possibility, at least, 
of infection from the sheep's blood, but the authors’ conviction of the occurrence 
of “hereditary infection" leads them to suppose that in the larvae and newly 
hatched “keds 5 ' the flagellates are present in a cryptic stage, which is stimu¬ 
lated to final development by the ingestion of sheep’s blood. 
In order to throw some light on the relationship between C. meiophagia and 
the sheep, I have made attempts artificially to infect mice with them. 
In the course of these experiments I became interested in this question in 
general, and have extended the experiments to other vertebrates, using 
flagellates parasitising other insects also. These experiments are recorded in 
the second part of this paper, together with some observations on the vitality 
of C. melophagia in. vitro. 
II. EXPERIMENTS ON ARTIFICIAL INFECTION OF DIFFERENT 
VERTEBRATES WITH INSECT FLAGELLATES. 
i 
The possible pathogenicity of insect flagellates of the Herpetomonas type 
for vertebrates in which they do not naturally occur is interesting from several 
points of view. It may, on the one hand, throw light on the question of 
the phylogeny of the pathogenic haemo-flagellates—the trypanosomes and 
leishmanias; on the other hand it may serve to elucidate to a certain degree 
the role of insects as carriers and intermediate hosts in certain diseases in which 
such have not hitherto been found, e.g. in Kala-azar and Oriental Sore. 
That trypanosomes and leishmanias have originated from purely insect 
flagellates which, by association of their hosts with vertebrates have gradually 
adapted themselves to life in the latter, is a hypothesis which, as far as 
trypanosomes are concerned, finds confirmation in the fact that the latter 
usually pass part of their life-cycle in some insect, the insect stages being 
similar to the forms of purely insect flagellates of the Herpetomonas group. 
As regards leishmanias, there is no direct evidence as yet of their connection 
with such forms, the only evidence of their relationship to the Herpetomonads 
being the flagellate forms they produce in cultures, the fact that they develop 
into flagellates in the stomachs of certain insects (Wenyon, 1911), and the very 
rare occurrence of such forms in the body of their vertebrate host (cf. Wenyon, 
1914, 1915). 
