IN connection with previous correspondence on the mode of production of the contractile vacuole in Protozoa (NATURE, vol. cvi., pp. 343, 376, 441), I find that it is, in point of fact, Prof. Marcus Hartog to whom the credit of the osmotic view is to be given. In a communication to the British Association in 1888 (Rep., p. 714) this observer pointed out that, owing to the semi-permeable surface membrane, substances in solution in the protoplasm of these organisms must attract water, which accumulates at a particular spot until it reaches the surface, breaks through the membrane, and escapes. The membrane spontaneously closes up as the distension is relieved. Prof. Hartog shows that if substances such as sugar or potassium nitrate are dissolved in the outer water to a sufficient osmotic concentration, the production of the vacuole ceases. The paper was reprinted in Ann. Mag. Nat. Hist., Sec. 6, vol. iii., p. 64 (1889). The theory was worked out in more detail by Degen (Bot. Zeit., vol. lxlli., abt. 1, 1905), and is explained by Prof. Hartog in his article on Protozoa in the Cambridge Natural History (1906), p. 15. My knowledge of Stempell's paper was derived from an abstract in which the osmotic aspect was chiefly emphasised. On reference to the original I find that this part of the process is obscured by a number of complicated subsidiary hypotheses.