A. Porter 
39 
active movements (Figs. 18, 19). Pseudopodia are withdrawn and the 
Amoeba gradually becomes passive (Figs. 18, 19, 20). The streaming 
movements of the general protoplasm are slowed, and following this, 
the contractile vacuole suspends action (Fig. 20). The protoplasm then 
contracts around the nucleus and the organism appears as a small, oval, 
uninucleate body. Externally, a cyst wall, probably secreted by the 
ectoplasm, is formed (Fig. 21). 
With one exception, the cysts I have observed are uninucleate. In 
one case, however, a fresh preparation was examined in which nuclear 
multiplication had occurred, five or six small, retractile daughter nuclei 
being present. I regret that I was unable to stain this particular cyst 
satisfactorily. 
Figs. 18—21. Stages of encystment; drawn from a single living A. cliironomi during a 
period of 45 minutes. 
Figs. 18, 19. Progressive withdrawal of pseudopodium and closure of contractile vacuole. 
Fig. 20. Organism has rotated. Contractile vacuole closed. The remaining small, blunt 
pseudopodium (ps.) took 20 minutes to withdraw entirely. 
Fig. 21. Encysted A. chironomi ; thick cyst wall (cy.) well seen. 
The cyst wall is thick and refringent, of a chitinoid nature, and has 
the characters usually associated with cysts. There seem to be no 
spines, warts or markings upon the cyst wall. Encystment, as before 
mentioned, occurs naturally in the posterior one-sixth of the gut 
of the Ghironomus, and is induced by the concentration of the intestinal 
contents. The natural cysts have precisely the same appearance as 
those artificially produced. They are from 12/* to 20/* long and 9/* to 
12/* broad. 
The time necessary for artificially induced encystment varies. I have 
once seen encystment take place in only two minutes, but in the form 
figured (Figs. 18—21) the entire process of cyst formation occupied 
forty-five minutes. Of this time the withdrawal of the process ps. in 
Fig. 20 took about twenty minutes. During this interval rotation of 
the organism occurred. 
