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Pneumocystis carinii 
their vermicular appearance, becoming rounded. All the merozoites 
observed had the same kind of staining reactions and there was no 
morphological or biological character that could be interpreted in 
terms of sex. 
The degree of infection of the mouse was slight. 
The mode of transmission of the parasite has not yet been determined. 
It seems probable that Pneumocystis carinii passes from host to host 
by the contaminative or casual method. The presence of a cyst wall 
around an organism is usually a protective adaptation for ensuring 
survival outside the body of the host. The occurrence of such a cyst 
in P. carinii suggests that the parasite leaves the host in the encysted 
condition, probably by way of the mucus in the oral or nasal 
secretions. 
The zoological position of P. carinii is not yet fully established. 
The experiences of a number of workers have shown that the organism 
is unconnected with any trypanosome. In my own case, the mouse 
harboured no trypanosomes in its blood, nor did I find Leucocytogregarina 
musculi to be present. Coccidia were absent from the gut. 
Delanoe suggested that the organism is possibly a Coccidium of the 
lung of its host. At present there is lack of knowledge of differentiated 
sexual forms, and until further stages of the life-cycle are known, the 
definite systematic position of Pneumocystis carinii cannot be 
stated. 
Before concluding, I should like to refer again to Leucocytogregarina 
musculi Porter 1908, parasitic in the leucocytes and internal organs of 
mice. At one time, I thought that Pneumocystis carinii might possibly 
be a part of the schizogonic phase of the former organism, though 
Pneumocystis appears to be somewhat small in size compared with the 
known scliizonts and merozoites of Leucocytogregarina musculi. The 
recent suggestion of A. C. Stevenson (1915) that L. musculi and Klossiella 
muris are stages of one organism is so far not supported by facts and 
seems to me improbable. 
Conclusions. 
Pneumocystis carinii has been found in the lungs of a mouse in 
England. The cysts are spherical or oval and are 4p, to 6-5 g in diameter; 
eight merozoites are finally formed within them. The organism appears 
to be an independent parasite of the lungs of certain vertebrates, and 
to be unconnected with any trypanosome. 
