This scheme has the merit of recognising the fact that the carbon dioxide does not wholly arise from the products of decomposition of hexosephosphate, nor from its direct fermentation. The function assigned to the phosphate is that of removing dihydroxyacetone and thus preventing it from inhibiting further conversion of hexose into triose, according to the reversible reaction C{6}H{12}O{6} ⇌ 2 C{3}H{6}O{3}. This however appears to be quite inadequate, since, on the one hand, the fermentation of glucose proceeds quite freely in presence of as much as 5 grams per 100 c.c. of dihydroxyacetone [Harden and Young, 1912], and on the other hand alcoholic fermentation appears not to proceed at all in the absence of phosphate (see p. 55). This forms the chief objection to the theory in its present form. The slow rate at which [p109] glyceraldehyde is fermented also affords an argument against the validity of Lebedeff's view, but this may possibly be accounted for to some extent by the fact that glyceraldehyde is a strong inhibiting agent so that it might be more rapidly fermented if added in a more dilute condition. The unfermented glyceraldehyde cannot be recovered from the solution and nothing is known as to its fate except that it readily gives rise both to lactic acid and glycerol [Oppenheimer, 1914, 1, 2]. Evidently the reaction between glyceraldehyde and yeast-juice is by no means a simple one. THE PYRUVIC ACID THEORY. The third stage of Lebedeff's theory postulates the intermediate formation of pyruvic acid. This idea immediately suggested itself when it became known that yeast was capable of rapidly decomposing /a/-ketonic acids with evolution of carbon dioxide [see Neubauer and Fromherz, 1911, p. 350; Neuberg and Kerb, 1912, 4; Kostytscheff, 1912, 2]. This scheme has been differently elaborated by different workers. According to Kostytscheff it involves (1) the production of pyruvic acid from the hexoses, a process accompanied by loss of hydrogen; (2) the decomposition of pyruvic acid into acetaldehyde and carbon dioxide; and (3) the reduction of the acetaldehyde to ethyl alcohol. (1) C{6}H{12}O{6} = 2CH{3}·CO·COOH + 4[H]. (2) 2CH{3}·CO·COOH = 2CH{3}·CHO + 2CO{2}. (3) 2CH{3}·CHO + 4H = 2CH{3}·CH{2}·OH. 1. As regards the production of pyruvic acid from the hexoses by yeast, the only direct evidence is afforded by the experiments of Fernbach and Schoen [1913] who have obtained a calcium salt having the qualitative properties of a pyruvate by carrying out alcoholic fermentation by yeast in presence of calcium carbonate, but have not yet definitely settled either the identity of the acid or its origin from sugar. Pyruvic acid is, however, very closely related to several substances which are intimately connected both chemically and biochemically with the hexoses. Thus lactic acid is its reduction product, CH{3}·CO·COOH → CH{3}·CH(OH)·COOH, + 2H