358 PROTOZOOLOGY 



unstained cysts might be dead or uninfectious is unknown. But as 

 Wenyon and O'Connor wrote, "if we accept the eosin test as a 

 criterion and regard all unstained cysts as living, the error in judg- 

 ment will be on the safe side." Root found neutral red in 1:10,000 

 dilution to give a slightly larger proportion of stained cysts than 

 eosin. Frye and Meleney's (1936) comparative study leads one to 

 look upon this method as a fairly dependable one. 



(b) Cultivation test. Improved cultural technique now brings 

 about easily excystment of viable cysts in a proper culture medium. 

 For example, Yorke and Adams (1926) obtained in 24 hours "a 

 plentiful growth of vegetative forms" from cysts in Locke-egg-serum 

 medium (p. 717). Snyder and Meleney (1941) note recently that the 

 excystation does not take place in various culture media unless liv- 

 ing bacteria were added or oxygen concentration of the media was 

 decreased. Animal infection method has not been used much, as 

 experimental animals (cats) show individual difference in suscepti- 

 bility. Some of the published results are summarized below. The 

 testing method used is indicated by: a for eosin test or h for cultiva- 

 tion test and is given after the name of the investigators. 



1. Cysts in faeces kept in a covered container. All cysts disap- 

 peared in 3 days at 37°C.; at 27-30°C. half of the cysts found dead 

 by the 4th and all dead by the 9th day (Kuenen and Swellengrebel ; 

 a). Alive for 3 weeks (Thomson and Thomson; a). Remain un- 

 changed for several weeks if kept "cool and moist" (Dobell). All 

 dead within 10 days at 16-20° or 0°C. (Yorke and Adams; b). 



2. Cysts kept in water emulsion. All alive on the 9th, but almost 

 all dead on the 13th day (Kuenen and Swellengrebel; a). Viable for 

 25 days (Thomson and Thomson; a). Cysts in running water for 15 

 days, excysted in pancreatic juice (Penfield, Woodcock and Drew). 

 Viable for 30 days (Wenyon and O'Connor; a) ; for 5 weeks (Dobell) ; 

 for 153 days (Boeck; a). Alive for 10 and 17 days at 16-20° and 0°C. 

 respectively (Yorke and Adams; 6); for 3. 10, 30, and 90 days at 

 30°, 20°, 10° and 0°C. respectively (Chang and Fair; b). 



3. Cysts in relation to high temperatures. Cysts are killed at 

 68°C. in 5 minutes (Boeck; a); at 50°C. in 5 minutes (Yorke and 

 Adams; 6). Dipping in boiling water for 5-10 seconds kills the cysts 

 (Kessel; a). 



4. Cysts in relation to desiccation. Desiccation kills cysts instantly 

 (Kuenen and Swellengrebel; Wenyon and O'Connor, Dobell, etc.). 

 Therefore, the cysts carried in dust are most probably not viable 

 under ordinary circumstances. 



