GENERAL CHARACTERS OF LIVING ORGANISMS 27 



by the organism in the characteristic activities (food- 

 seeking, etc.) required for its individual maintcnanrc 

 and the perpetuation of its kind. 



From this general point of view the simplest cases 

 are the most instructive; e.g., that of a single yeast 

 cell or bacterium introduced into a nutrient medium. 

 The organism grows and divides until eventually in 

 place of the single cell there are thousands. Evidently 

 the material of these additional cells comes from the 

 surrounding medium, certain constituents of which arc 

 transformed into the living material or protoi)lasm. 

 The total quantity of material in the whole system, 

 organism plus culture-medium, is unaltered; but its 

 condition has undergone a profound change. A typical 

 nutrient solution for yeast (Pasteur's solution) contains 

 sugar and various salts (NaK tartrate, chlorides, phos- 

 phates, and sulphates of Na and K) together with water 

 and oxygen. From these relatively simple materials are 

 built up proteins, lipoids, fats, and other complex bodies; 

 not only are these characteristic substances synthesized 

 but they are distributed or arranged (partly in solid 

 form) in a definite and constant manner so as to give 

 rise to numerous complex and uniformly constituted 

 systems, the yeast cells. Each of these, once formed, 

 becomes the seat of further transformations of the same 

 kind; and by a repetition of this process the non-living 

 material of the medium is progressively transformed 

 into living protoplasm. The transformation is constant 

 and specific, chemically, structurally, and ])hysiologically ; 

 ''heredity" receives here its simplest manifestation.* 



* Cf. my paper, "Heredity from a Physico-Clicinical Point of \i<\v, 

 Biological Bulletin, XXXIV (191 8), 65. 



