VARIETIES OF GENERATION. 



941 



of propagation. Then, finally, there develops the sexually mature form (imago, 

 for example butterfly), which yields, through the union of sperm and ovum, the 

 fecundated initial form in the developmental process. Metamorphosis occurs 

 extensively among insects, either with many (holometabola) or with few inter- 

 mediate stages (hemimetabola) ; likewise in other arthropods and in some worms 

 (for example trichina). The sexually mature male and female descendants of 

 the trichina are set free in the intestine, where they enter into sexual union and 

 live for but a short time. They are known as intestinal trichina. They produce 

 many eggs, which penetrate the muscular tissues of the host and constitute the 

 larva. The encapsulated sexually immature muscle-trichinae, which may remain 

 quiescent for more than thirty years, are the pupce, and these, when ingested in 

 the living state by another suitable organism, develop in the intestine of the latter 

 into sexually mature and active individuals. Among vertebrates, metamorphosis 

 occurs also in the amphibia (frogs) , and among fish in lampreys (petromyzon) . 

 The alternation of generations (metagenesis) exhibits in common with meta- 

 morphosis the series of externally different forms in the process of development. 

 It differs materially from metamorphosis, however, in the circumstance that the 

 animal can multiply asexually in one or the other of the stages. The final stage 

 alone then exhibits the sexual reproduction. Medically, the most important 



I 12 II 



FIG. 352. Heads of Taenia solium (I) and Taenia mediocanellata (II) and Mature Proglottids of each (i, 2). 



example is furnished by the tapeworms (teniae). The sexually mature, hermaph- 

 roditic individual, with hundreds of testicles, vasa deferentia, penises, ovaries, 

 yolks, shell-glands, vaginas and uteruses (Fig. 351), is the protoglottis (tapeworm- 

 segment), which becomes detached and evacuated with the feces; it is motile 

 and occasionally continues to grow. From an ovum (Fig. 345), rendered capable 

 of reproduction by self-impregnation, there results an elliptical embryo, provided 

 with six booklets. This gains entrance with food into the intestine of another 

 animal, whence it penetrates into the tissues and there develops into the third 

 stage, the bladder-worm cysticercus, cenurus, echinococcus. Within this 

 vesicle there develops only one (cysticercus, Fig. 347) or several (cenurus) 

 short-pedicled tapeworm-heads; or within the vesicle there develop at first 

 numerous daughter- vesicles, and within those many heads (echinococcus, Fig. 

 349). For further development the bladder-worm must be consumed alive by 

 another being. Then, the tapeworm-heads (scolex) attach themselves to the 

 wall of the intestines by means of their hooklets or suckers and by budding form 

 a chain of numerous segments (Fig. 3 50), each developed link of which constitutes 

 a sexually mature offspring of the tenia. The most important tapeworms are 

 as follows: Tasnia solium is found in the intestine of man. Its bladder-worm, 

 cysticercus cellulosae (Fig. 352), occurs in swine, seldom in man. Taenia medio- 

 canellata is found in the intestine of man (Fig. 352); its bladder-worm in the 



