CHEMICAL STRUCTURE AND METABOLISM 95 



internal as well as external, are bathed by sea water. These animals have 

 no blood of their own, but sea water serves the same purpose, conveying 

 to the cells such needed materials as oxygen and carrying away waste 

 products of cell metabolism. As we intimated earlier, the "living stuff," 

 protoplasm itself, has as its basis a salt solution. Apparently protoplasm 

 originated in an environment of salt water. And apparently, also, pro- 

 toplasm can exist only in an environment of salt water. Furthermore, 

 the salt concentration and the proportions in which the various salts occur 

 can seemingly vary only within fairly narrow limits if protoplasm is to 

 survive. 



We may picture, then, what probably occurred when in the course of 

 their evolution animals became so complex in structure that some parts 

 were no longer bathed directly by sea water. These parts would need a 

 carrying agent to bring such essentials as oxygen and digested foodstuffs 

 to them and to remove waste products. So some of the sea water was 

 "bottled up" within the organism and became the blood. Mechanisms for 

 propelling the blood throughout the body developed. In this way even the 

 cells of innermost tissues of the body continued to live in an environment 

 like that of the sea water in which life began. 



If our interpretation is correct, animal bloods are similar in salt composi- 

 tion and concentration because life originated in an environment of salt 

 water having about this composition and concentration and can continue 

 to exist only in such an environment. The blood provides the cells with 

 an "internal environment" reminiscent of the external environment pre- 

 vailing when life began. Although some cells have become adapted for 

 life in fluids of quite diff'erent concentrations, it is still true on the whole 

 that "la fixite du milieu interieur est la condition de la vie libre" (Claude 

 Bernard). Constancy of internal environment is a necessary condition for 

 life. This fact is reflected in another of the fundamental similarities unit- 

 ing varied members of the animal kingdom. 



Osmotic Regulation 



The necessity for maintaining a relatively constant internal environ- 

 ment has presented organisms with serious problems as they have invaded 

 difl'ering external environments. A major factor is the phenomenon of 

 osmosis. Some membranes, including the living membranes of plants and 

 animals, are said to be difl"erentially permeable; some substances pass 

 through them more readily than do others. If, for example, a differentially 

 permeable membrane has salt solution on one side of it and pure water 



