198 EMBRYOLOGY 



(Fig. 96 H). Ordinarily only the neck portion of the 

 scolex, which is immediately attached to the head, is said 

 to be inclnded in the adult Avorm, whereas all the rest dis- 

 integrates. Leuckart obsei-ved such young stages of Tcenia 

 solium, which moved about freely in the intestine of their 

 host by extending their suckers like arms and again retract- 

 ing them. They were no longer so much elongated as is 

 the case after their evagination from the Cysticercus (com- 

 pare Fig. 96 F), but had only a short stump-like appendage. 

 The formation of the segments takes place in such a way 

 that the terminal segment is the oldest, and the youngest 

 ones are always interpolated in the vicinity of the head. 

 Growth and formation of segments take place so rapidly 

 that the tapeworm soon attains a great length, and the 

 posterior segments become detached from the others. With 

 the faeces of the host they reach the outside world, where 

 they are encountered creeping about slowly. 



In the younger proglottides nothing can as yet be recognized of the 

 genital apparatus. This arises out of the parenchymatous tissue in the 

 central ijart of the proglottis, which to a certain extent still remains in 

 an embryonic condition, as the result of a more compact massing of the 

 cells. This cell-mass, which is at first spherical, later elongates and is 

 differentiated in such a manner that three cords of cells occupying the 

 longitudinal axis of the worm can be distinguished. F. Schmidt, who 

 studied these conditions in Bothriocephalus latus, found that these three 

 cell-cords produced the sexual ducts, which, therefore, begin to develop 

 earlier than the germ glands. In consequence of a luxuriant cell-proli- 

 feration, these cords increase in length, the ventral one, which is earliest 

 differentiated, becoming the vagina, the dorsal becoming the vas deferens, 

 and the extensive cell-mass lying between them becoming the uterus. 

 In proglottides of Bothriocephalus which lie about 50 cm. behind the 

 head, the sexual ducts have become connected with the surface of the 

 body, and the sexual openings can be recognized. About 10 cm. behind 

 the head the genital fundaments appear sim^^ly as a dark longitudinal 

 streak in the middle line of the segments. The germ glands and vitel- 

 laria likewise arise from the parenchymatous tissue, but independently 

 of the ducts, with which they become connected by means of cords of 

 parenchymatous cells, which afterwards become hollow. 



General Considerations. — The course of development in the 

 Cestodes has met with various interpretations. The older conception, 

 established by Steenstrup, looks upon it as a true alternation of 

 generations. According to this theory, inasmuch as the scolex buds 



