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contained in endothelial cells. A cyst wall, of varying degrees of thick¬ 
ness in different specimens, was usually present. 
Older cysts showed two (Fig. 2), four (Fig. 3) or eight (Fig. 5) nuclei. 
In some cases, the nucleus was in process of division into two, and 
a strand connected the two daughter nuclei. These nuclei were not 
always equal in size, and one of them at times simulated a blepharoplast. 
Binary fission, three times repeated, resulted in the production of an 
Figures 1—8. 
Pneumocystis carinii, as seen in the lung of a wild mouse in England, x 3000 
1. Uninucleate parasite. 2. Binucleate form. 3. Organism with four nuclei. 
4. Parasite showing the progress of the division of the nucleus into eight. 
5. Commencement of cytoplasmic segmentation. 6, 7. Merozoites differentiating. 
8. Pneumocyst containing eight fully formed vermicular merozoites. 
eight-nucleate organism (Fig. 4). Cytoplasmic cleavage into eight 
portions followed (Figs. 5, 6), and eight merozoites were gradually 
produced (Figs. 7, 8). Fully mature cysts measured 5p, to 6-5/x in 
diameter. 
Each merozoite was a small vermicular body with a single nucleus. 
The length of the merozoite was T5p to 2-5/a, and its breadth about 
0-5p.. The free merozoites were slow in movement, and rapidly lost 
