DEVELOPMENT OF THE ORGANISM 479 



insects (Sec. 315), tunicates (Sec. 340), and amphibians (Sec. 402). 

 Metamorphosis may be varied in degree, but the terms complete and 

 incomplete are applied only to metamorphosis in the insects, depending 

 upon whether development does or does not include a pupal stage. 

 Sometimes more stages than those in complete metamorphosis are 

 observed; this phenomenon is called hyperrnetamorphosis. Larval life 

 may be accompanied by the development of characteristic larval organs 

 which later disappear and are lacking in the adult. Such adaptations 

 usually characterize individual types, are termed cenogenetic, and are to 

 be distinguished from larval characteristics which are ancestral and 

 phylogenetic in character. 



495. Potential Immortality of Germ Cells. — Germ cells possess a 

 potential immortality, since any germ cell has the capacity under proper 

 conditions to take part in the production of another individual, and this 

 may continue for an indefinite number of generations. Nevertheless, 

 they perish in enormous numbers, since many eggs are never fertilized 

 and a greater number of sperm cells never find an egg cell with which to 

 unite. In contrast to germ cells, somatic cells present no possibility of 

 life beyond the lifetime of the individual of which they are a part. The 

 distinction between germ cells and somatic cells, or, more exactly, between 

 germ plasm and somatoplasm, was emphasized by Weismann (Sec. 118). 

 He also stressed the independence of the germ cells and likened the body, 

 or soma, to a vehicle for the nourishment and transmission of germ cells. 

 The hereditary units, which determine the possibilities open to the ani- 

 mal, are passed from generation to generation in the germ cells, while 

 in the various types of somatic cell, under the environmental conditions 

 which surround each, are realized and manifested such of these possi- 

 bilities as, taken together, equip the individual with its characteristic 

 features. 



