Immunity in Unicellular Organisms 13 



stitutes a condition favourable to infection by the Chytridia. Whilst [15] 

 the growth of Volvocina, Euglenae and their allies is almost always 

 interfered with by very destructive parasitic epidemics, the ciliated 

 Infusoria, capable of seizing and digesting lower organisms, may be 

 cultivated and flourish for a very long period. Thus Balbiani 1 has 

 watched one of his cultures of Paramaecium aurelia multiply and 

 thrive in splendid condition for 14 years in succession. Now these 

 Infusoria readily adapt themselves to ordinary water untreated to 

 render it more hygienic. Such water swarms with all sorts of lower 

 organisms, among which are the Chytridia and numerous Bacteria, but 

 the Paramaeda and Infusoria in general feed upon these organisms 

 and contribute largely to the purification of the water. Almost the 

 whole body-contents in a ciliated Infusorian is made up of a digestive 

 protoplasm into which the captured Bacteria and other lower 

 organisms are conveyed ; the nutrient particles becoming sur- 

 rounded by transparent vacuoles, in which the ingested organisms 

 are killed and digested. The food contained in the vacuoles circulates 

 in the endoplasm of the Infusoria by means of the streaming 

 movements of this layer. The digestive vacuoles become filled with 

 a fluid having a distinctly acid reaction. Formerly, in order to 

 demonstrate this reaction, Infusoria were allowed to ingest small 

 granules of blue litmus which after a certain time became more 

 or less intensely red ; but the use of aniline colours has much 

 simplified the study of digestion in microscopic organisms. By 

 introducing a solution of alizarin sulpho-acid into a liquid con- 

 taining Infusoria, the yellow staining (characteristic of the acid 

 reaction) of the digestive vacuoles can be readily made out. When 

 the Infusoria ingest small clumps of alkaline substances, stained 

 violet by this reagent, the vacuoles take on a red tint, indicating 

 the acidity of their contents 2 . Another aniline colour, neutral red 

 (Xeutralroth), introduced into microscopical technique by Ehrlich 3 , 

 enables us to demonstrate the acid reaction in the digestive vacuoles 

 even within a few minutes. Thus, in Paramaeda treated with a dilute 

 solution of this reagent, the digestive vacuoles at once assume the 

 deep rose tint, characteristic of an acid reaction. This coloration is 

 observed during the life of the Infusorian, but immediately after death [16] 



1 Arch, d'anat. microsc., Paris, 1898, t. n, p. 528. 



2 Le Dantec, "Recherches sur la digestion intracellulaire," Lille, 1891, p. 53. 



3 Ehrlich u. Lazarus, " Die Anaraie," in Nothnagel's " Specielle Pathologic u. 

 Therapie;' Wien, 1898, Bd. vin, I ter Theil, S. 85; also "Pathology of the Blood," 

 authorised English translation, Cambridge, 1900, p. 125. 



