DEVELOPMENT 



425 



duces 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 



FIG. 374. Alternate generation : a, b, c, ova of an Acaleph (Ckrysaora); d,e,f, Hydras; 

 g, h t Hydras with constrictions; z, Hydra undergoing fission; k, one of the separated 

 segments, a free medusa. 



from eggs. Often, as in the fresh-water hydra, the two 

 kinds of generations are alike in appearance. The 

 process is as widespread as asexual reproduction, being 

 found mostly in sponges, ccelenterates, and worms. It 

 is also found in certain Crustacea and insects. The 

 name is sometimes limited to cases where the two kinds 

 of generations differ in form. 



3. Growth and Repair 



Growth is increase of bulk, as Development is increase 

 of structure. It occurs whenever the process of repair 

 exceeds that of waste, or when new material is added 

 faster than the tissues are destroyed. There is a specific 

 limit of growth for all animals, although many of the 

 low cold-blooded forms, as the trout and anaconda, seem 

 to grow as long as they live. After the body has at- 

 tained its maturity, i.e., has fully developed, the tissues 

 cease to grow ; and nutrition is concerned solely in sup- 



