255 
THE OCCURRENCE OF PNEUMOCYSTIS CARINII 
IN MICE IN ENGLAND. 
By ANNIE PORTER, D.Sc. Bond., F.L.S., 
Beit Memorial Research Felloiv, Quich Laboratory, 
University of Cambridge. 
(With 8 Text-figures.) 
Introduction. 
A fertile source of error in protozoology is to be found in the 
overlooking of the occurrence of mixed infections in a host. Two 
organisms, each independent of the other, may happen to coexist in the 
intestine of an insect or the blood of a vertebrate, the result being that 
the investigator may confuse stages in the life-cycles of parasites which 
are not related to each other. This source of error has led to un¬ 
warranted generalisations which have impeded scientific progress for 
years. Two examples at once come to mind—the trouble wrought by 
Schaudinn in 1904 in confusing the life-cycles of the protozoa found 
in the little owl, and the impediment put in the way of progress by the 
omission of certain workers to recognise that Crithidia grayi was not 
a part of the life-cycle of a trypanosome. The existence of mixed 
infections was overlooked in each case. 
Pulmonary Cysts. Historical. 
A third example of confusion of identity occurred in connection 
with Trypanosoma ( Schizotrypanum) cruzi, wherein Chagas (1909, 1911) 
described two forms of scliizogonic development of the parasite. One 
form, occurring in many tissues, is generally accepted, but around the 
other, which is said to occur in the lungs, controversy has arisen. 
Certain trypanosomes in the lungs of vertebrate hosts were said to 
become round and then eight-nucleate, giving rise to eight vermicular 
merozoites. These daughter forms were considered by Chagas to be 
