CELLS AND THEIR ASSOCIATION 47 



reason to believe that a more vigorous and enduring 

 organism is produced. It is a means of rejuvenation. 

 Conjugation of free-living cells seems a prophecy of 

 sex as realized in the higher varieties of plants and 

 animals. In these we know that the union of the 

 two germ-cells gives rise to an organism which we call 

 young, meaning that it has powers of growth and 

 development which the parents have largely lost. 



We ought now to compare somewhat fully the situa- 

 tion of a cell that exists alone and one which acts as a 

 member of the great community making up the body of 

 one of the larger animals. In the first place, a direct 

 consequence of the size of such a body is that the great 

 majority of the cells are submerged and surrounded by 

 their fellows instead of maintaining immediate relations 

 with the outside world. Special provision must be 

 made for bringing food to such cells and relieving them 

 of waste. These purposes are served in the higher 

 animals by liquid media, the blood and the lymph. 



The blood is confined to a system of vessels in which it 

 moves steadily in one direction. Only those cells which 

 Kite the vessels are actually bathed by the blood. All 

 other cells and this means the vast majority are 

 removed from direct contact with the blood but have 

 around and between them the second fluid, the lymph. 

 From this they draw nutriment and oxygen; to it they 

 discharge carbon dioxid and other products of Iheir 

 activity. The lymph adjacent to any one cell is a very 

 limited quantity and if there were no provision for its 

 renewal its usefulness would soon be at an end. But 

 the blood is flowing close by in capillaries whose thin 

 walls scarcely impede the passage of dissolved gases and 

 other materials between the blood and the lymph. 

 By a continuous exchange between the two fluids the 

 lymph is relieved of cell waste and held to a standard 

 composition as regards oxygen and food. 



One of the striking facts we note when we compare 

 free-living and associated cells is expressed by the term 



