396 SENESCENCE AND REJUVENESCENCE 



period seems to conflict with the conclusion reached in the 

 present chapter that the formation of gametes is a feature of 

 relatively late stages in the life history of the individual. This 

 conflict, however, is apparent rather than real. Each larval genera- 

 tion has a life history of its own not essentially different from that 

 of other animals: during this period it undergoes progressive dift'er- 

 entiation and growth, but the rate of growth decreases and the 

 larva finally dies, apparently of old age. I have determined the 

 changes in susceptibility to cyanides of two of the larval generations 

 of certain species and have found that a marked and rapid decrease 

 in susceptibility occurs in each generation and that the early stages 

 of each generation show a much higher susceptibiHty than the late 

 stages of the preceding generation. This means that each genera- 

 tion undergoes a rapid senescence and that rejuvenescence occurs 

 during each reproduction, but there is some evidence that progressive 

 senescence from generation to generation also occurs to some extent. 

 During the earlier stages of the life of a larva the cells which 

 later become germ cells undergo division and so increase in number, 

 but they do not become mature and begin independent development 

 into new individuals until a relatively late larval stage of larval 

 life is reached. The period of reproduction through the germ cells 

 is in fact a feature of advanced age in the life of the larva. The cells 

 resemble eggs in possessing a low metabolic rate before beginning 

 development because they are parts of a physiologically old body, 

 and it is probable that the occurrence of a maturation division with 

 the formation of a polar body is connected with this condition (see 

 pp. 353-56). What we commonly cafl the life history of these 

 worms is then in reality a series of life histories with alternating 

 periods of senescence and rejuvenescence. Each period of senes- 

 cence is accompanied in its later stages by reproduction through 

 cells which resemble parthenogenic eggs more or less closely, but 

 only in advanced age of the final generation, the adult form, do 

 sexual maturity and fertilization occur. Certain other points in 

 these life histories are of interest in connection with the problem of 

 the fife cycle, but this brief consideration is perhaps sufficient to 

 show that the pecuhar larval reproduction of these species is a 

 feature of advanced age Hke gametic reproduction in other forms. 



