1913.] t Colpoda cucullus from its Resting Cysts. 



429 



optimum temperature for excystation, owing to there being no incubators at 

 other temperatures, but it lies close to 30° C. 



Meunier* obtained active Colpoda two hours after moistening resting cysts, 

 and Fabref also obtained active forms in the same time. 



Influence of Alkaline, Acid, and Neutral Media on Excystation. — The ordinary 

 1-per-cent. hay-infusion used for cultures of Colpoda and other protozoa is 

 made slightly alkaline in reaction to litmus by the addition of caustic soda 

 solution and contains O'Ol per cent. XaOH. Colpoda cv.cvllus excysts very 

 freely in this liquid and also in neutral 1-per-cent. hay-infusion. When, 

 however, the hay-infusion was made slightly acid in reaction to litmus, 

 excystation appeared to be completely inhibited, and from these preliminary 

 trials the inference was tentatively drawn, that excystation was inhibited in 

 an acid medium. 



It was necessary, however, to determine more accurately the critical 

 percentage strength of acid aud alkali at which excystation is inhibited. 



XaOH. — 1-per-cent. hay-infusion was put up in 10 c.c. lots, each with a 

 different percentage strength of XaOH, starting at - 01 per cent, and going 

 up in hundredths of 1 c.c. to - 2 per cent. 



Excystation was rapid and free in all the cultures below - 15 per cent. 

 XaOH. At this strength, however, only a few were active after two hours' 

 incubation and these were apparently rather uncomfortable. At 016 per 

 cent, one or two were trying to excyst, 017 per cent, showed one or two 

 revolving within their endocysts but unable to get out quickly, at 018 per 

 cent, there was no motion, though the contractile vacuoles in one or two 

 were dilated, 0T9 per cent, gave the same appearance as 018 per cent., 

 and at 0'2 per cent, there was no indication whatever of excystation. 



After 21 hours' incubation active forms were found in all, up to and 

 including 018 per cent, and 019 per cent., but none were active in 02 per 

 cent. This then is the critical percentage strength of XaOH which inhibits 

 excystation, whilst 018 per cent, may be taken as the critical strength for 

 short-period incubation. 



HC1. — Tor hydrochloric acid a number of 10 c.c. lots of 1-per-cent. hay- 

 infusion were put up, each with a different percentage strength of HC1, 

 starting at - 001 per cent, and going up to O'Ol per cent, in thousandths of 

 1 c.c. and then in hundredths of 1 c.c. up to 01 per cent. 



After one hour's incubation at 30° C. there were many active in all the 

 cultures up to 0*01 per cent. At 0'08 per cent, there were a few active, 



* Meunier, V., " Sur la resistance vit. des Kolpodes ' encystes,' " ' Compt. Eend.,' 1865, 

 vol. 61. 



t Fabre-Domergue, P., loc. cit. 



