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, b, c, ova of an Acaleph (Chrysaora) ; d,e,f, Hy- 

 dras ; g, 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 individual 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 



