2. That such a solution can serve for the synthesis of all the compounds of living yeast cells is due to the fact that it contains the sugars. From the sugars organic acids can be formed and these with ammonia (which was offered in the form of ammonium tartrate) may give rise to the formation of amino acids, the “building stones” of the proteins. It is thus obvious that the synthesis of living matter centres around the sugar molecule. The phosphates are required for the formation of the nucleins, and the work of Harden and Young suggests that they play also a rôle in the alcoholic fermentation of sugar. Chlorophyll, under the influence of the red rays of light, manufactures the sugars from the CO₂ of the air. This makes it appear as though life on our planet should have been preceded by the existence of chlorophyll, a fact difficult to understand since it seems more natural to conceive of chlorophyll as a part or a product of living organisms rather than the reverse. Where then should the sugar come from, which is a constituent of the majority of culture media and which seems a prerequisite for the synthesis of proteins in living organisms? The investigations of Winogradsky on nitrifying,[8] sulphur and perhaps also on iron bacteria have to all appearances pointed a way out of this difficulty. It seemed probable that there were specific micro-organisms which oxidized the ammonia formed in sewage or in the putrefaction of living matter, but the attempts to prove this assumption by raising such a nitrifying micro-organism on one of the usual culture media, all of which contained organic compounds, failed. Led by the results of his observations on sulphur bacteria it occurred to Winogradsky that the presence of organic compounds stood in the way of raising these bacteria, and this idea proved correct. The bacteria oxidizing ammonia to nitrites were grown on the following medium; 1 gm. ammonium sulphate, 1 gm. potassium phosphate, 1 gm. magnesium carbonate, to 1 litre of water. From this medium, which is free from sugar and contains only constituents which could exist on the planet before the appearance of life, the nitrifying bacteria were able to form sugars, fatty acids, proteins, and the other specific constituents of living matter. Winogradsky proved, by quantitative determination, that with the nitrification an increase in the amount of carbon compounds takes place. “Since this bound carbon in the cultures can have no other source than the CO₂ and since the process itself can have no other cause than the activity of the nitrifying organism, no other alternative was left but to ascribe to it the power of assimilating CO₂.”[9] “Since the oxidation of NH₃ is the only source of chemical energy which the nitrifying organism can use it was clear _a priori_ that the yield in assimilation must correspond to the quantity of oxidized nitrogen. It turned out that an approximately constant ratio