Cited by M. Greenwood and E.R. Saunders, 'On the Rôle of Acid in Protozoan Digestion', Journal of Physiology 16 (5-6) (1894), pp. 441-467.
Description:

'The possibility that the substance of animal cells may give rise within itself to accumulations of acid fluid, even when these cells are not highly differentiated, has been considered indeed by many earlier writers, and their statements have gained a degree of familiarity by repetition. But if we except a paper by M. Metchnikoff [note: 'E. Metchnikoff. Annales de l'Inst. Past. 1889, p. 25.'], which contains a discussion of the reaction of plasmodia to ingested litmus, we find that the records which bear on this question are often incidental and unsupported by the details of experiment.' (441)

 

'In point of time... , this work [of le Dantec] was preceded by the paper in which Metchnikoff describes the ingestion of blue grains of powdered litmus by different species of plasmodia and subsequent change of the blue colour to red, pointing out that, as the substance of the plasmodia is clearly alkaline, this change indicates the secretion of an acid fluid. He records a comparable result with certain of the larval phagocytes in the tail of Triton toeniatus, and, noting that in the same cell a reddened grain of litmus may coexist with a vacuole in which blue grains lie, he draws the inference that the acid secretion may be localized even in the relatively small mass of a single wandering cell.' (442-443)

 

'while Metchnikoff and le Dantec are alike in emphasizing the intracellular secretion of acid fluid by relatively undifferentiated protoplasm, neither of them deals at any length with the part played by this acid in the solution of nutritive material as it is carried out within the cell; the point which we, on the other hand, were most anxious to examine is the transformation undergone by acid indicators as associated with successive phases of the digestive process.' (444)