486 GENERAL PHYSIOLOGY 



But this sharp separation of assimilation and dissimilation on the 

 one side, and growth and atrophy on the other, can scarcely be main- 

 tained, at least in so far as the former are conceived to be based 

 upon purely qualitative, the latter upon purely quantitative changes 

 of living substance. The formation of living substance takes place 

 only with the help of living substance already present. Only 

 where such substance already exists can new masses of it be formed. 

 This is true even of the plant-cell, in which the living substance is 

 produced in great measure from purely inorganic materials. It 

 must be concluded from this that in growth the biogen molecule 

 attracts to itself from the food the elements necessary for the 

 formation of living substance and combines them chemically, and, 

 therefore, it is changed qualitatively in growth. The general 

 tendency of proteids, and likewise of the cyanogen-containing 

 groups of atoms hypothetically present in the biogeri molecule, to 

 polymerisation, as Pfliiger has already emphasised, allows us to 

 understand this growth by chemical union. On the other 

 hand, atrophy is only conceivable as taking place by means 

 of chemical decomposition, that is, by a qualitative change of 

 the living particles. But even if we can, and must, dis- 

 tinguish the regeneration of certain parts of the biogen molecule 

 from the reformation of whole biogen molecules, and, likewise, the 

 separation of single groups of atoms from the complete decomposi- 

 tion of the molecule, chemical changes are always present, which 

 are directed to either the construction or the destruction of complete 

 biogen molecules. Regeneration is only a part of the process of 

 the formation of a new biogen molecule, and, likewise, the splitting- 

 off of certain groups of atoms is only a part of the phenomenon of 

 decomposition. In an hypothesis upon the nature of assimilation, 

 Hatschek ('94) has also established a relation between this process 

 and growth. He assumes that in growth the simple molecule of 

 living proteid continually attracts elements to itself from the food 

 until it has become a polymeric molecule ; it then breaks down into 

 simple molecules, and the latter gradually develop chemically anew 

 into a polymeric molecule by the union of the necessary atoms and 

 the groups of atoms, and so on. In other words, Hatschek 

 likewise sees in growth a chemical process, which does not differ 

 fundamentally from regeneration. After all these considerations 

 it appears advantageous to employ the conceptions of assimila- 

 tion and dissimilation in the more general sense, including therein 

 the formation of new and the disappearance of old molecules, and 

 to give to them the above exact wording : 



Assimilation comprises all those transformations that lead up to the 

 construction of bio gens, dissimilation all those that extend from the 

 decomposition of biogens down to the complete formation of the 

 excretion-products. 



It is, however important to examine somewhat more in detail 



