20 GENERAL INTRODUCTION 



masses, each of which possesses one or more differentiated 

 portions called a nucleus. 



Such a Mass with its Nucleus is a Cell. A cell may be defined 

 as the elementary unit of all organisms, no matter how simple 

 or how complicated they may be. Every organism begins its 

 individual history as a cell separated from a preexisting organ- 

 ism. From time to time this cell (ovum, spore, etc.) divides 

 itself into two or more parts, each of which in due time divides 

 again, the resulting divisions in every case forming complete 

 cells. In the protozoa the daughter cells separate, and each 

 leads an independent existence, but in many-celled animals they 

 remain connected and become dependent upon one another. 



An histological differentiation takes place as the animal develops, 

 so that groups of cells are formed which are totally different 

 in appearance, and results in tissue and organ formation. 

 They take on different functions, pari passu, and one group 

 of cells will perform a certain work for the good of the entire 

 economy. They thus lose their individuality and become 

 dependent upon one another. This is known as the physio- 

 logical division of labor. 



The exact molecular structure of living matter is unknown, 

 but there is no doubt that it is of very great complexity. It 

 differs from dead protoplasm in its unstable, labile nature, 

 reacting to an enormous number of substances which are 

 indifferent to dead protoplasm. It manifests a continual 

 tendency to undergo changes, while dead protoplasm, if pro- 

 tected from external agencies, can be kept indefinitely. The 

 nitrogen-containing oxidation products derived from the two 

 are radically different. Those from living matter uric acid, 

 creatin, adenin, xanthin, guanin, etc. are all characterized 

 by the possession of the cyanogen group, CN. This group 

 is one of great internal energy, so that compounds containing 

 it have a marked tendency to undergo dissociation. This 

 is especially the case in the presence of oxygen. It is a well- 

 known fact that cyanogen compounds also have the property 

 of polymerization that is, of combining with compounds 

 having a structure like their own, so as to form more complex 

 combinations. By such a process living matter becomes less 

 and less stable, until the instability reaches its acme, when 

 the compound undergoes a breaking-down process, resulting 



