PROTOZOA 565 



have recently fed voraciously. She has also come to the con- 

 clusion that the lowered _pH often found in cultures containing 

 many cysts and young amoebae is probably the result rather than 

 the cause of encystment, the acidity being due to the decomposi- 

 tion of the residual cystoplasm of the mother amoebaj after the 

 cysts have been liberated. 



Other more elaborate methods of cultivating amoebae have been 

 devised by Sch^effer {Carnegie Jnstit., xxiv, 1926) to imitate running 

 water and other conditions in Nature, but for ordinary laboratory 

 purposes the above described closed mass cultures are satisfactory. 



1111. General Notes on Cultures in Aqueous Media. The 



acidity of a culture containing algae can, of course, be reduced 

 and its ^H proportionally raised : — 



1. By stirring and shaking or keeping it in a shallow vessel 

 with a relatively large surface exposed to the air, thus enabling 

 the CO2 to escape. 



2. By placing the culture in a bright light, since during photo- 

 synthesis CO2 in the water is broken up. 



3. By the addition of minute quantities of alkalies, such as 

 NaHCOg, NaOH, etc. 



Conversely, the pH soon falls if the culture is made in a test- 

 tube, where a comparatively small surface of the medium is 

 exposed to the air, for then the COg produced accumulates in 

 the liquid ; or COg may be passed into the medium. (When 

 there is only a small amount of medium in a test-tube, for example, 

 a suitable indicator may easily be made to change colour by simply 

 breathing into the tube (Saunders, Proc. Camb. Phil. Soc.y i, 

 1925, p. 24.9).) Other acids may be added instead of COg, such as 

 very dilute hydrochloric acid. Taylor has used tartaric acid for 

 increasing the acidity of amoeba cultures but finds that it 

 encourages detrimental bacteria ; she has had some success in 

 eliminating undesirable organisms from cultures by temporarily 

 making the pH unsuitable to them ; or by adding chemicals, e.g., 

 ferrous sulphate for getting rid of a tiresome blue-green alga. 



It is quite impossible to deal adequately here, with the very 

 numerous methods devised for keeping isolation or mass cultures 

 of various free-living protozoa, from the time of Maupas {A^'ch. 

 Zool. Exper., 1888) onwards. 



Everyone knows that Paramoecium, for instance, can very easily 

 be cultivated in hay infusions. Other media successfully used are 

 infusions of decaying leaves, suspensions of malted milk, boiled flour 

 water, as well as water containing wheat. The optimum pVi is found 

 by Saunders {Proc. Camb. Phil. Soc, i, 1925, p. 249) to be 7-8-8. 

 Taylor keeps Actinosphaerium successfully in aquaria with high pH 

 by feeding them on the rotifer, Chydorus or others of the numerous 

 animals they will devour. 



For Spirostomum ambiguuni, Saunders {ibid., 1924) finds media of 

 pHS and above are toxic. Therefore, vmless a " soft " water is used for 



