144 



GENERAL PRINCIPLES OF ZOOLOGY. 



reproduction alternate in a quite definite rhythm with each other. 

 Such a development is called alternation of generations in the wider 

 sense, and of this two special forms are distinguished: meta- 

 genesis, or alternation of generations in the narrower sense (pro- 

 gressive alternation of generations), and heterogony (regressive 

 alternation of generations). 



Progressive Alternation of Generations. Metagenesis. Alter- 

 nation of generations in the narrower sense, or metagenesis, is the 

 alternation of at least two generations, of which one reproduces 

 only asexually, by division or budding, the other either exclusively, 

 or at least to a great extent sexually. The first generation is called 

 the nurse, the second the sexual animal. The reproduction of 

 hydromedusae furnishes the best example (fig. 91). The nurses 



FIG. 91 . Bougainvillea ramnsa. (From Lang.) ft, hydranths (nurse) which have given 

 rise to medusa-buds (m/c) ; m, separated medusa, Margelis ramosa (sexual animal). 



here are the polyps, which, united with one another usually in 

 numbers into a colony, never produce sexual organs, but bud sexual 

 animals, the medusce. The medusae are altogether unlike the 

 polyps, being much more highly organized, and freely motile; only 

 very rarely have they preserved the asexual mode of reproduction; 

 on the other hand, they develop eggs and spermatozoa, from which 

 the non-motile nurses, the polyps, develop. This example shows 



