246 FOUNDATIONS OF BIOLOGY 



conjugation 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 dif- 

 ference is that, in the case of Paramecium, the products 

 of division of the animal which has conjugated (EXCONJU- 

 GANT) separate as so many independent cells, all of which are 

 alike and, in later generations, capable of conjugation; while 

 all the products of division of the fertilized egg of multi- 

 cellular forms remain together as a unit and become differ- 

 entiated 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, if 

 somewhat fancifully, it is stated that after conjugation in 

 Paramecium we have the period of greatest cell vigor, or 

 youth, followed by maturity when the cells are ripe for con- 

 jugation again, and in the absence of conjugation 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 conjugation. 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 uni- 

 cellular forms because fertilization 'rejuvenates' the cell, and 

 the cell and the individual are one and the same. With the 

 origin of multicellular forms, involving the segregation of 

 soma from germ, death became possible, and was established 

 it is the 'price paid for the body/ (Figs. 115, 135.) 



Suggestive as is this comparison and contrast and it is 

 not without some justification the cardinal fact remains 

 that recent work has demonstrated that Paramecium, under 

 favorable environmental conditions, can continue reproduc- 

 tion indefinitely; at least for fourteen years and some ten 



