Pneumocystis carinii 
merozoites of T. cruzi. Later, Carini (1910) found similar rounded 
encysted structures in the lungs of rats in Sao Paulo, Brazil, the hosts 
being infected with T. lewisi. Yianna (1911-12) found pneumocysts 
in the lungs of animals infected experimentally with T. gambiense, 
T. equinum, T. congolense and T. equiperdum, whence the cysts were 
considered to be stages in the life-cycles of those trypanosomes. 
However, M. and Mme Delanoe (1912), working at the Institut Pasteur, 
Paris, found similar structures in the lungs of rats immune to T. lewisi. 
They doubted whether the cysts were an integral part of the life-cycle 
of the trypanosome, and considered that, they might be separate 
organisms for which the name Pneumocystis carinii was suggested. 
Their subsequent work in 1914 has confirmed the opinion that P. carinii 
is a separate entity, for it has been found in animals not infected with 
trypanosomes. Chagas (1913) revised his statements regarding the 
schizogony of Trypanosoma cruzi, and was supported by Aragao (1913). 
Carini (1914) has now accepted Pneumocystis carinii as an independent 
organism. 
Pidmonary cysts have also been found in rabbits, dogs, cats, lambs 
and goats, and possibly in man. They seem to have been observed 
more particularly in Brazil and in France. Coles (1914) found pneu¬ 
mocysts in one rat in the south of England. 
Pneumocystis carinii in Mice. 
So far as I am aware, P. carinii has not been recorded from mice, 
at any rate in England. During last year, I examined a wild mouse 
killed accidentally at Cambridge. The body was well nourished and 
in good condition. No parasites were found in the peripheral or heart 
blood. On very finely teasing a small portion of one of the lungs, and 
examining the preparations either fresh and unstained, or stained intra 
vitam with methylene blue, a number of small bodies corresponding 
to P. carinii were seen. Portions of organs other than the lungs 
yielded no parasites. Fourteen other mice since examined for pulmonary 
cysts have been negative. 
The various stages of Pneumocystis, as seen in a mouse, may be 
briefly described: 
The youngest cysts seen were spherical or oval bodies, 4p to 5p in 
diameter. They were uninucleate and their cytoplasm was finely 
granular or slightly alveolar. Chromatinic structures other than the 
nucleus were not seen (Fig. 1). The cysts were either free or were 
