GENERAL EMBRYOLOGY 



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different modes of reproduction alternate with each other. This is called 

 alternation of generations in the wider sense, and of this two special forms 

 are distinguished: metagenesis (progressive alternation of generations), 

 and heterogony (regressive alternation of generations). 



Metagenesis. Alternation of generations in the narrower sense, or 

 metagenesis, is the alternation of at least two generations, one reproducing 

 only asexually, by division or budding, the other 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 examples (fig. 94). The nurses here are the polyps, which 



Fir,. 94. Bougainvillea ramosa (from Lang). /;, hydranths (nurse) which have 

 given rise to medusa-buds (mk) ; m, separated medusa, Margelis ramosa (sexual 

 animal). 



usually united into a colony, never produce sexual organs, but bud sexual 

 animals, the medusce. The medusae are unlike the polyps, being much 

 more highly organized, and freely motile; only very rarely do they repro- 

 duce asexually; on the other hand, they develop eggs and spermatozoa, 

 from which the non-motile nurses, the polyps, develop. This example 

 shows that, in alternation of generations, there is not only a difference in 

 the mode of reproduction, but usually in addition, a difference in form 



