DEVELOPMENT. 



213 



in number) with scalloped edges. Next, the pile breaks 

 up into separate segments, which are, in fact, so many dis- 

 tinct animals ; and each turning over as it is set free, so 

 as to bring the mouth below, develops into an adult Me- 

 dusa, becoming more and more convex, and furnished 

 with tentacles, circular canals, and other organs exactly 

 like those of the progenitor that laid the original egg. 



Here we see a Medusa producing eggs which develop 

 into stationary forms resembling Hydras. The Hydras 



FIG. 178 Alternate Generation: a, 6, c, ova of an Acaleph (Chrysaora) ; d,e,f, Hy- 

 dras ; 0, h, Hydras with constrictions ; i, Hydra undergoing fission ; k, one of the 

 separated segments, a free Medusa. 



then produce not only Medusae by budding in the manner 

 described, but also other Hydras like themselves by bud- 

 ding. All these intermediate forms are transient states 

 of the Jelly-fish, but the metamorphoses cannot be said to 

 occur in the same individual. While a Caterpillar becomes 

 a Butterfly, this Hydra-like individu&l produces a number 

 of Medusae. Alternate generation is, then, an alternation 

 of asexual and sexual methods of reproduction, one or 

 more generations produced from buds being followed by 

 a single generation produced from eggs. Often, as in 

 the fresh-water Hydra, the two kinds of generations are 

 alike in appearance. The process is as wide- spread as 

 asexual reproduction, being found mostly in Sponges, 

 Coelenterates, and Worms. It is also found in certain 



