REPRODUCTION 



process is known as pwdogenesis ("young-generation"). 

 It is found particularly in the platodes (trematodes) and 

 some of the insects (larvae of cecidomyca and other flies). 



In a large number of lower animals and plants sexual 

 and asexual generation regularly alternate. Among the 

 protists we find this alternation of generation in the 

 sporozoa; among the metaphyta in the mosses and 

 ferns ; and among the metazoa in the cnidaria, platodes, 

 tunicates, etc. Often the two generations differ con- 

 siderably in shape and degree of organization. Thus, in 

 the mosses the asexual generation is the spore-forming 

 moss capsule {sporogonium) , while the sexual is the moss 

 plant with stalk and leaves (culmus). In the case of the 

 ferns, on the other hand, the latter is spore-forming and 

 monogenetic, while the thallus-formed, simple, and small 

 fore-germ (prothalHum) is sexually differentiated. In 

 most of the cnidaria a small stationary polyp is devel- 

 oped out of the ovum of the free-swimming medusa, and 

 this polyp, in turn, generates by budding medusas, 

 which reach sexual maturity. In the tunicates (salpa) 

 a sexual social form alternates with an asexual solitary 

 form; the chain-salpa of the former are smaller and 

 differently shaped than the large individual salpa of the 

 latter, which again generate chains by budding. This 

 special form of metagenesis was the first to be observed, 

 as it was in 1819 by the poet Chamisso, when he sailed 

 round the world. In other cases (for instance, in the 

 closely related doliolwm) a sexual generation alternates 

 with two (or more) neutral ones. The explanation of 

 these various forms of alternating generations is given 

 in the laws of latent heredity (atavism), division of 

 labor, and metamorphosis, and especially by the bio- 

 genetic law. 



While in real metagenesis (alternation of generations 

 in the strict sense) the asexual generation propagates by 

 budding or spore-formation, this is done partheno- 



2S3 



