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 lieterogony (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 sexvial animal. The reproduction of 

 hydromedusae furnishes the l)est example (fig. 01). The nurses 



Fir. 'M.—B(ni{iainviUi(i rirniMN.r. iKmrn LanuM /i, liydrautlis (nui-si-i n-liirli liavr i;iven 

 riaci to medusa-buds {ink); in, separated iiirdusa, Miinirlis iiiiiitisii (sexual aiiiuiall. 



licre are the polyps, which, united with one another nsnallv in 

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

 animals, the medusa'. The meditsie are altogether unlike the 

 ])olyps, being mucli more highly organiz.ed, and freely nnitile; only 

 very rarely ha\"e tliey preserved the asexual mode oF reprdduetion ; 

 on the other band, tliey develop eggs and spermatozoa, frcun Avhich 

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



