178 



PROTOZOOLOGY 



When a cyst encounters a proper environment, redifferentiation 

 takes place within the cyst. Various organellae which characterize 

 the organism, are regenerated and reformed, and the young tropho- 

 zoite excysts. The emerged organism returns once more to its trophic 

 phase of existence. Experimental data indicate that excystment 

 takes place under conditions such as addition of fresh culture me- 

 dium (Kiihn, 1915; Rosenberg, 1938), hypertonic solution (Ilowai- 

 sky, 1926), distilled water (Johnson and Evans, 1941), organic in- 

 fusion (Mast, 1917; Beers, 1926; Barker and Taylor, 1933), and bac- 

 terial infusion (Singh, 1941; Beers, 1946a) to the culture medium. 

 Change in pH (Koffman, 1924), lowering the temperature (John- 

 son and Evans, 1941) and increase in oxygen content (Brand, 1923; 

 Finley, 1936) of the medium have also been reported as bringing 

 about excystment. Excystment in Colpoda cucullus is said to be due 



Fig. 74. Encystment of Euglypha acanthophora, X320 (Kiihn). 



to specific inducing substances present in plant infusion (Thimann 

 and Barker, 1934; Haagen-Smit and Thimann, 1938). Experiment- 

 ing with two soil amoebae, "species 4 and Z," Crump (1950) found 

 that the excystment in species Z took place without the presence of 

 bacteria and regardless of the age of the cysts, but species 4 excysted 

 only in the presence of certain bacteria (Aerobacter sp. or "4036") 

 and the excystment diminished with the age of cysts. Crump sug- 

 gested that the two strains of bacteria appeared to produce some 

 material which induced excystment in Amoeba species 4. In Tillina 

 magna, Beers (1945) found, however, the primary excystment-in- 

 ducing factor to be of an osmotic nature and inducing substances, 

 a secondary one. 



As to how an aperture or apertures are formed in the cyst wall 

 prior to the emergence of the content, precise information is not 

 yet on hand, though there are many observations. In the excyst- 

 ment in Didinium and Tillina, Beers (1935, 1945, 1945a) notes that 



