256 ANIMAL BIOLOGY 



is an example, a large amount of experimental breeding has been 

 carried out on Paramecium and its allies. The earlier results seemed 

 to demonstrate conclusively that Paramecium can divide only a 

 limited number of times, say a couple of hundred, after which the 

 cells die from exhaustion or senile degeneration unless fertiliza- 

 tion takes place. In other words, it was believed that periodic 

 rejuvenation by fertilization is a necessity for the continuance 

 of the life of the race. And therefore, so the natural conclusion 

 ran, protoplasm is unable to grow indefinitely; there is an inherent 

 tendency for the destructive phases of metabolism to gain ascend- 

 ancy over the constructive, and fertilization serves to maintain or 

 restore the youthful condition and thus secure the continuance of 

 the race. 



In this connection, the life history of Paramecium from one 

 period of fertilization to the next is often compared to the life of a 

 multicellular organism from its origin as the fertilized egg, through 

 youth and adult life to old age. The striking difference is that, in 

 the case of Paramecium, the products of division of an animal 

 which has conjugated (exconjugant) separate as so many inde- 

 pendent cells, all of which are alike and, in later generations, 

 capable of fertilization; while all the products of division of the 

 fertilized egg of multicellular forms remain together as a unit and 

 become differentiated for particular functions in the individual, 

 except a few, the germ cells, which retain the power of forming 

 new individuals. Pushing this comparison a little further, it is 

 stated that after fertilization in Paramecium we have the period of 

 greatest cell vigor, or youth, followed by maturity when the cells 

 are ripe for fertilization again, and in the absence of fertilization — 

 and only then — the onset of old age, and death. Thus death has 

 no normal place in the life history of Paramecium, for all the cells 

 at the period of maturity are capable of fertilization. On the other 

 hand, in multicellular forms only some of the cells, the germ cells, 

 retain this power - - the somatic cells have paid the penalty of 

 specialization and must die. Thus death of the individual except 

 by accident does not occur among unicellular forms because ferti- 

 lization 'rejuvenates' the cell, and the cell and the individual are 

 one and the same. With the origin of multicellular forms, involv- 

 ing the segregation of soma from germ, death became possible, and 

 was established — it is the 'price paid for the body.' (Figs. 154, 

 C, D; 180.) 



