358 
Dientamoeba fragilis 
(1907) that an unencysted Trichomonas can pass through the stomach, 
and infect a new host in this way. We can hardly suppose, however, 
that Dientamoeba can thus effect its entry into the human body; for 
its fragility and lack of resistance when outside the body constitute one 
of its most characteristic features^. It occurs to us that there is another 
possible explanation of the apparent absence of cysts in these two 
organisms. It is well known that Entamoeba histolytica, which is probably 
a true human parasite, encysts in the typical manner within its human 
host. But it is also facultatively parasitic in other animals—for example, 
in the cat. In this host, however, the amoeboid form of the parasite 
alone occurs. The amoebae appear to have lost their power of forming 
cysts. It seems to us conceivable that a similar phenomenon may possibly 
by presented by Dientamoeba and Trichomonas. It is possible that both 
are parasites belonging to some host other than man, but capable of 
existing and multiplying in the human bowel. On such a hypothesis 
we would suppose that both organisms undergo encystation^ in their 
normal hosts in the usual way, and that man becomes accidentally 
infected by ingesting their cysts. But in man, their foreign host, they 
have both lost—like E. histolytica in the cat—the power of forming 
cysts. 
We may add here that we have made several attempts to cultivate 
Dientamoeba on agar media, but, as we expected, without success. 
We have no reason to suppose that Dientamoeba is a pathogenic 
organism. It is true that we have found it chiefly in persons who have 
suffered from dysentery o'r chronic diarrhoea. This, however, ceases to 
have any significance when it is remembered that the vast majority of 
persons whose faeces we examine are so examined solely because of 
their intestinal ailments. Up to the present we have found only seven 
cases of infection with Dientamoeba, out of a very large number of 
persons whose stools we have examined. And although six members of 
our series are persons who have suffered from diarrhoea or dysentery 
abroad, on the other hand the remaining case (D. 1) is a person who has 
never left the British Isles, and who has never suffered from dysentery, 
persistent diarrhoea, or other intestinal ailment. We have endeavoured 
for some time past to examine, whenever possible, the stools of healthy 
1 Trichomonas hominis, on the other hand, will sometimes survive apparently un¬ 
changed for a long time after it has left the human body. On one occasion we found 
living and apparently healthy trichomonads in a sample of human faeces every time it 
was examined for four weeks after it was passed. 
- Some species of Trichomonas undoubtedly form cysts. These were first described 
by one of us (C. D., 1908, 1909) in T. batrachorum., a species occurring in amphibians. 
