46 HUMAN PHYSIOLOGY 



half-cells, one furnished by the male and one by the 

 female. Thus the animal of the higher sort has two 

 parents, four grandparents, eight ancestors in the next 

 generation, the numbers soon becoming enormous as we 

 reckon backward. The 'bacterium, on the other hand, 

 has but one ancestor in a generation. 



The case of the one-celled animal forms is not so simple 

 as may have appeared from the unqualified statements 

 that have been made. Close observation of these 

 orders of life has shown that, while the multiplication 

 of cells by cleavage is the common process among them, 

 in many instances the fusion of two cells into one (con- 

 jugation) is a possibility. When it takes place there is 

 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 

 line 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. 



