420 SENESCENCE AND REJUVENESCENCE 



that these regressive processes are going on, progressive changes 

 are occurring and new structural features are beginning to appear. 

 In some embryos these do not become visible or conspicuous until 

 the regressive changes are far advanced, while in others, such, for 

 example, as certain armehds and moUusks, in which larval forms 

 differentiate very early in development, they may begin to appear 

 during the first few divisions following fertilization, or some of the 

 structural features of the egg may be carried over into the larva. 

 In short, both the degree and rate of morphological regression, as 

 well as the degree and rate of rejuvenescence during early stages, 

 vary greatly in different forms. 



LARVAL STAGES AND METAMORPHOSIS 



In many animals the form hatching from the egg is widely 

 different, both in structure and in behavior, from the adult, and is 

 known as a larva: sooner or later this form undergoes either a 

 gradual or a somewhat abrupt transformation or metamorphosis 

 into the adult form. The question as to the nature of larval 

 metamorphosis and the internal and external conditions which 

 determine it has been much discussed, and various hypotheses 

 have been advanced. Here, however, the purpose is only to present 

 a few suggestions rather than to attempt extended discussion. 



In the first place the term "larva" is a loose biological term with 

 little physiological significance. The larva is merely a form differ- 

 ent from the adult and appearing before it in the life history. But 

 the larva of an annehd which develops during the first few cell 

 divisions after fertiHzation is very different from the larva of an 

 insect or a frog which appears only after thousands of divisions and 

 extensive differentiation. The larval form may represent an 

 earlier or a later stage in the developmental history. 



In many invertebrates, e.g., in the annehd Nereis, the larval 

 form develops during the period of rejuvenescence. So far as I 

 have been able to determine, the eggs or embryos of all species 

 in which the larval form arises very early possess a strongly marked 

 axial gradient and individuation progresses rapidly, while in those 

 where the larval period occurs at a later stage the gradient is much 

 less clearly marked in early stages and develops only gradually. 



