THE MECHANISM OF LIFE 485 



2. Bio tonus 



Now that we have become acquainted with the simplest schem- 

 atic expression of the elementary vital process in the construction 

 and destruction of biogens, we must consider more in detail certain 

 metabolic relations that result from these, and we must define 

 certain conceptions which are important in clarifying our ideas 

 upon metabolism. 



It will be recalled that two phases are distinguished in meta- 

 bolism, assimilation and dissimilation. By assimilation is understood 

 the capacity of living substance to construct its like continually 

 from the ingested food-stuffs ; by dissimilation, the capacity to 

 decompose continually into the products excreted by it. In ac- 

 cordance with the above considerations, this conception can be 

 formulated more exactly as follows: assimilation comprises all those 

 transformations that lead up to the construction of biogens, dis- 

 similation all those that extend from the decomposition of biogens 

 down to the complete formation of the excretion-products. 



Such an exact definition of these two fundamental conceptions 

 of the theory of metabolism is necessary, for, when we glance at 

 the history of the theory, we find that they have been employed 

 with very different meanings. Assimilation, which originally 

 signified in a very general sense the formation of living substance in 

 the organism out of non-living food, has been employed by botanists 

 in a very special way. Plant physiology in large part still means 

 by assimilation exclusively the synthesis of starch from water and 

 carbonic acid in the chlorophyll-bodies of the green plant-cell. 

 This narrow conception has gradually been widened in animal phy- 

 siology, and the term has been employed not only for the synthesis 

 of the first organic product, but also for the construction out of the 

 ingested food-stuffs of the more complex compounds of living sub- 

 stance, especially those that are characteristic of every form of cell, 

 the proteids. In contrast to this latter use, Ewald Hering ('88) 

 has conceived the word in a narrow sense, and in a small but sug- 

 gestive work has sharply separated assimilation from growth. By 

 the former he understands only the qualitative chemical change of 

 particles already present ; in other words, the completion of the 

 particles up to the maximum of their constitution ; under growth, 

 on the other hand, he includes not qualitative changes, but only a 

 quantitative increase of the particles present. In addition to this 

 Hering has created the conception of dissimilation and placed it 

 beside that of assimilation, finding between dissimilation and 

 atrophy a difference corresponding to that between assimilation 

 and growth ; the qualitative change associated with the separation 

 of certain substances from the particles present he terms dissimila- 

 tion, and the quantitative diminution of the particles, atrophy 



