802 



SCIENCE 



[N. S. Vol. XXIX. No. 751 



work and growth commences. Many of the 

 lower forms of life can not feed upon the 

 proteins. This is true of the yeast cell. 

 These cells grow rapidly when placed in a 

 solution of sugar and nitrates, but proteins 

 must be broken up by putrefactive bacteria 

 before yeast organisms can feed upon them. 

 Indeed, many of the cells in the body of 

 man can not feed upon the complex pro- 

 teins, which must be split up into simpler 

 groups by the digestive enzymes before the 

 cell molecules can absorb and assimilate 

 them. Even the carbohydrate starch must 

 be hydrated and its elements rearranged as 

 the constituents of a more labile molecule 

 before it can become a source of energy in 

 muscle. Solutions of proteins injected into 

 the blood of man are poisonous, but the 

 same substances, after being properly split 

 up, are valuable sources of energy, and 

 some are essential to the continuance of 

 those functions that constitute life. 



The atomic groups cast out from the 

 living molecule are not altogether waste 

 products, for among them are the specific 

 secretions which act upon the extramolec- 

 ular substance, and fit it for absorption 

 and assimilation. Many of these substances 

 have a reversible action. The colon bacillus 

 will feed at one time on solutions of highly 

 complex proteins, the secretion splitting up 

 the large molecule into smaller groups, thus 

 acting as an analytical agent. The same 

 bacillus will grow in a medium that con- 

 tains available nitrogen only in the form of 

 amino-acids, its secretion acting as a syn- 

 thetical agent and building up complex 

 bodies out of simpler ones. It is by means 

 of the specific secretion that the living cell, 

 or the molecules of which it is composed, 

 feeds, or, in other words, breaks up the 

 pabulum placed within its reach into groups 

 suitable for absorption or assimilation. In 

 case of fixed cells the food stuff must be 

 brought to the cell or within close range of 



it before it can be prepared for assimilation. 

 Life in a cell molecule ceases when food is 

 withdrawn, or when the secretion is want- 

 ing, or from any cause made inactive. No 

 living molecule can continue to function 

 when new material is wanting. Each liv- 

 ing, acting molecule has a selective action 

 in the assimilation of new hiaterial, and this 

 is quite indicative of metabolism being a 

 chemical or intramolecular process. It 

 might be supposed that a living molecule 

 is a highly complex body, possibly with 

 many basic and many acid groups. In the 

 process of metabolism an acid group is 

 cast out and this enters into one of the 

 molecules of the foodstuff, setting free 

 another acid group which takes the place 

 of the group cast out. In this way the 

 living molecule feeds, regenerates itself, 

 and supplies itself not only with new mat- 

 ter but with energy. These movements are 

 rhythmical and continue in the same tempo 

 so long as conditions remain the same. It 

 may be that groups of different sizes are 

 cast out and replaced from time to time, 

 smaller groups being thrown off more, and 

 larger ones less, frequently. We have rea- 

 son for believing that in many living mole- 

 cules there are at least two kinds of metab- 

 olism which occur simultaneously, but in 

 different tempo. The more frequent con- 

 sists in casting out from the living molecule 

 relatively small groups consisting lai'gely of 

 oxidation forms of carbon and hydrogen. 

 The less frequent is a nitrogenous metabol- 

 ism in which urea or some antecedent of 

 this substance is eliminated and is replaced 

 by fresh material. It is possible that indi- 

 vidual molecules are broken up beyond 

 repair in the nitrogenous metabolism, the 

 worn-out one having already reproduced its 

 successor by polymerization. The tempo 

 with which these processes of metabolism 

 occur may be altered by changing the ex- 

 ternal conditions. AVhen the speed is too 



