388 SENESCENCE AND REJUVENESCENCE 



In other animal groups in which agamic reproduction is a more 

 or less characteristic feature of the life cycle — certain families of 

 annelids, the bryozoa, and the tunicates — it is in general true 

 that agamic precedes gametic reproduction in the life history, and 

 in some of these forms, notably in certain annelids, agamic repro- 

 duction may apparently continue indefinitely under certain con- 

 ditions without the attainment of sexual maturity. 



Among the higher invertebrates, and among the vertebrates, 

 the definite character and internal determination of the life history 

 become in most cases even more apparent. In many forms, as in 

 most of the insects, development ends in a single period of gametic 

 production followed by death. In many other forms, after ma- 

 turity is once attained, the production of gametes is periodic or 

 continuous and the animal may live for a long time and may also 

 undergo extensive growth, as do most of the mollusks after the 

 first period of sexual maturity. In such cases growth, as well as 

 gamete production, appears to be periodic, and the formation of 

 gametes follows, at least in most cases, the growth period. 



Periodicity of this sort in the organism is commonly associated 

 with periodicity in the environment, e.g., with seasonal or other 

 periodic changes. The environmental periodicity may determine 

 slight alterations of senescence and rejuvenescence as perhaps in 

 the case of the mollusks, where growth periods ending with or 

 followed by gamete formation occur or in other cases the activity 

 of the sexual organs may be directly influenced by nutritive con- 

 dition, temperature, etc. 



That the vertebrates pass through a definite developmental 

 history, with sexual maturity as a comparatively late stage, and 

 that this history is primarily determined by factors within the 

 organism rather than environmental conditions is sufficiently 

 evident. Here agamic reproduction does not occur, except in a 

 few cases in early embryonic stages, and the life history is without 

 the complications which arise in lower forms to prevent, balance, 

 or retard progressive development. Even among the vertebrates, 

 however, the appearance of sexual maturity may be hastened or 

 retarded by the character and amount of the food and by various 

 other environmental conditions. The tadpoles of frogs and sala- 



