THE MECHANISM OF LIFE 479 



their transformation can be considerably augmented by slight in- 

 tlucncos from the outside, as tlie excitation of metabolism by 

 stimuli clearly shows. Since, however, metabolism constitutes the 

 real vital process, it is seen at once that life depends directly upon 

 the existence of these labile complexes of atoms. We are, there- 

 fore, justified in examining these significant substances more in 

 detail and investigating their nature somewhat further. 



In searching after them we can best start from the decomposition- 

 products excreted in metabolism. It is here found that among 

 other substances, such as carbonic acid, water, and lactic acid, 

 which contain only the elements carbon, hydrogen and oxygen, com- 

 pounds also occur that contain nitrogen. The non-nitrogenous 

 decom[)osition-products may possiblj^ be derived from the decom- 

 position of carbohj'drates, fats, etc. ; but those containing nitrogen 

 can come only from the transformation of proteids or their 

 derivatives, for these are the sole bodies containing nitrogen that 

 are present in all living substance. This important fact directs 

 attention first to the proteids. 



That this is the right path becomes at once clear when the facts 

 concerning the proteids are recalled that have been mentioned in 

 the course of the previous considerations. These facts show with- 

 out doubt that the proteids stand at the centre of all organic life. 



It is an important fact that in all cases where large quantities 

 of reserve-substances, such as fat, starch, and glycogen, are not ac- 

 cumulated in cells, the proteids constitute by far the largest pai-t 

 of the organic compounds of living substance. This jjroves that 

 they must play a significant rSlc in the life of the cell. The dom- 

 inant position of the proteids among the chemical compounds of 

 living substance, however, is at once attested by the fact that they 

 are the onl)' substances that can be found in every cell without ex- 

 ception. It is a further fact that of all the more important sub- 

 stances in the cell the proteids and their compounds present the 

 highest complexity in chemical composition, they comprise the 

 largest number and variety of atoms in their molecules. The 

 known chemical relations of the non-nitrogenous organic sub- 

 stances, especially the carbohydrates and fots, to the proteids are 

 in harmony with this dominant position of the latter in living 

 substance ; for, so far as their history is known, those substances 

 either are consumed in building up the proteid molecule, or are 

 derived from the transformations of the latter. The former is, of 

 course, shown most clearlj- by plants, in which all organic com- 

 pounds are manufactured synthetically out of simpler inorganic 

 substances. In the cells of the green plant occurs the synthesis of 

 the first organic product, starch, out of cai-bonic acid and water. 

 This carbohydrate constitutes the organic basis from which the 

 proteid molecule is developed synthetically in a complex and still 

 partly unknown manner with the hel]) of nitrogenous and sulphur- 



