200 EMBRYOLOGY 



outward segmentation of the body. The entire animal, therefore, 

 corresponds to an individual with a segmented arrangement of the 

 organs, and not to an animal stock. The genital organs themselves 

 agree with those of the externally segmented Cestodes, and it appears, 

 therefore, as if we had before us in this case a condition which 

 corresponds to a more i^rimitive stage of the Cestodes. 



Although, from what has just been said, the course of development 

 of tapeworms would have to be considered as a metamorphosis, it is 

 nevertheless certain that in some foi-ms it rej^resents a real alternation of 

 generations. This is true of those forms in which more than one scolex 

 arises in the Cysticercus. The Cysticercus of Tmnia ccennrus produces 

 within itself a large number of tapeworm heads (about 500), and in the 

 bladder-w^orm of Tcenia echinococcus even daughter-vesicles are formed, 

 which in turn give rise to tapeworm heads. Here, where the embryo 

 produces many individuals, each one of which acquires the organization 

 of the tapeworm, there is unquestionably an alternation of generations. 

 The heads arise by means of budding in the Cysticercus ; they grow up 

 into segmented worms, and produce the sexual elements. In this case, 

 therefore, a sexual generation alternates with a non-sexual. The con- 

 ditions are still more complicated when there is interpolated a gene- 

 ration of daughter-vesicles, which bud from the parent vesicle and in 

 turn alone give rise to the heads. 



In conclusion we refer once more to the relation between Cestodes and 

 Trematodes. In addition to other anatomical characters, it is especially 

 the structure of the genital apparatus which brings the two groups very close 

 to each other. In both, the yolk glands, in addition to the germ glands, 

 contribute to the production of the eggs, which are therefore composed of 

 two kinds of cells. The development, too, proceeds in a homologous 

 manner, and shows, above all, a great similarity in the formation of the 

 embryonal membranes. In considering the further stages of the de- 

 velopmental cycle, we are led by such forms as ArcJdgetex (see supra, 

 p. 199), which must be considered as a sexually mature cysticercoid 

 larva, to the comparison of the Cysticercus stage of the Cestodes with the 

 Cercaria of the Trematodes, in which the caudal appendage of the 

 Cercaria is to be considered as the equivalent of the vesicular posterior 

 end of the Cysticercus (Glaus). In such an interpretation we must con- 

 sider the sporocysts and Rediffi as secondarily interpolated links of the 

 developmental cycle. They are essentially larval organisms, reproduc- 

 ing parthenogenetically, in which the organization and the form of the 

 Cercaria have secondarily undergone an alteration and partial degenera- 

 tion. In most of the Cestodes therefore the development from the egg to 

 the complete tapeworm must be considered as a simple metamorphosis ; 

 an alternation of generations being recognizable only in the Echinococcus 

 bladders, where the young forms (Cysticercus stage) possess the power 

 of reproduction by means of budding. The development of the Trema- 

 todes, on the contrary, ai:)pears under the form of heterogeny, in which 



