D. L. MACKINNON 
271 
Summary. 
1. Musca domestica and other non-biting flies frequenting similar 
feeding-grounds, are probably all liable to infection with a common 
flagellate. The great variability of this form is shown on comparing 
the flagellates in the larvae with those in the mature flies of Homa- 
lomyia, sp. and of Scatophaga lutaria. 
2. Infection is casual— i.e. by the mouth. In the case of the 
dung-flies examined, the larvae ingest faecal matter infected with 
herpetomonad cysts: the cysts develop into flagellates in the mid-gut, 
where they multiply with great rapidity: towards the close of larval 
life, when the larva stops feeding, they round up in the hind-gut, and 
are for the most part passed out as cysts. A few survive the pupal 
stage in a half-encysted condition, but it is probable that the infection 
of the adult fly is usually freshly acquired. The cycle in the fly is 
similar to that in the larva, and is in agreement with Patton’s account 
of Eerpetomonas muscae-domesticae. The parasite was never found in 
the ovaries or ova. Patton’s suggestion that the degree of infection 
depends directly on the number of cysts ingested, is borne out by the 
much higher rate of infection in the larvae than in the flies: this is not 
surprising when we remember the complete restriction of the feeding 
larva to the infected area. 
3. It is important to use some reliable cytological stain such as 
iron-baematoxylin as a control to Romanowsky stains where possible, 
seeing that very different results are sometimes given by the two 
methods. 
4. The apparent double flagellum is produced in the course of 
longitudinal division. The new flagellum grows up alongside the old, 
and is not merely split off from it: this is best seen in the flagellate 
from the larva, where the body of the organism usually divides at an 
earlier period than in the fly. Study of the living flagellate is necessary 
to a clear understanding of the process of division. 
5. I have seen no conjugation in the living herpetomonads. 
Occasionally flagellates were met with in Giemsa preparations devoid of 
a tropho-nucleus. Such individuals might be regarded as male gametes; 
it is more likely that they are simply degenerate forms. Sufficient 
consideration is not given to the possibility of degeneration in richly- 
nourished, rapidly multiplying protozoa, such as the trypanosomes anil 
their allies. 
