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 Medusa, 
becoming more and more convex, and furnished with ten- 
tacles, circular canals, and other organs exactly like those of 
the progenitor which laid the original egg (Figs. 178,195). 
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,/, 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, 
Ccelenterates, and Worms. It is also found in certain 
