1 6 THE ANIMAL PARASITES OF MAN 



followed the gradual metamorphosis from oncospheres to cystic 

 worms in all its details. 



In view of all the researches that were made, and which are too 

 numerous to mention individually, the idea that bladder-worms are 

 abnormal or only incidental forms had to be abandoned. Every- 

 thing pointed to the fact that in all cestodes the development is 

 divided between two kinds of animals ; in one the host, the adult 

 tapeworm is found ; while in the other, the intermediate host, we 

 find some form or other of an intermediate stage (cysticercus in 

 the broadest sense). The practical application of this knowledge is 

 self-evident. If no infected pork or beef is ingested, no tapeworm 

 can be acquired, and also the rearing of cysticerci in the human body 

 is prevented by avoiding the introduction of the eggs of tapeworms. 



Though these results were definitely proved by numerous 

 researches, yet they have been repeatedly challenged, notably by 

 ]. Knoch (1862) in Petrograd, who, on the basis of experiments, 

 sought to confirm a direct development without an intermediate host 

 and ciliated stage, at all events as regards Dibothriocephalns latns. 

 However, the repeated communications of this author met with 

 but little favour from competent persons, partly because the 

 experiments were conducted very carelessly, and partly because 

 their repetition on dog and man (R. Leuekart) had no results 

 (1863). It was only in 1883 that Braun was able to prove that the 

 developmental cycle of Dibothriocephalus latus is similar to that of 

 other Cestodes. The results obtained in other places by Parona, 

 Grassi, Ijima and Zschokke render any discussion of Kiichenmeister's 

 conclusions unnecessary. 1 Long after Knoch, a French author, 

 P. Megnin, also pleaded for the direct development of some 

 cestodes, and especially some taeniae. He (1879) also sought to 

 prove a genetic connection between the hookless and armed tape- 

 worms of mammals, but the arguments he adduced, so far as they 

 rest on observations, can be easily refuted or attributed to mis- 

 interpretation. Only one of these arguments is correct, namely, 

 that the number of the species of taeniae with which we are acquainted 

 is far larger than that of the corresponding cystic forms; out this 

 disparity alone cannot be taken as a proof of direct development. 

 It can only be said that our knowledge in this respect is deficient. 

 As a matter of fact, we have during recent years become acquainted 

 with a large number of cystic forms, hitherto unknown, belonging 

 to taeniae which have long been familiar. It must also be borne in 

 mind that no man in his lifetime can complete an examination for 

 bladder-worms of the large number of insects, for instance, which 



1 Refer to the collected literature under Dibothriocephahts latus, and the reply to Kuchen- 

 meister by Braun (" Ueber den Zwischenwirt des breit. Bandw." Wiirzb. : Stuber, 1886). 



