PULLULATION IN THE GENETIC CYCLE. 153 



must not allow this terminology to blind us to the acci- 

 dental nature of the distinction. 



5. The repetition of many successive gemmations or 

 what has been here termed pullulation must be considered 

 a much more variable feature in the reproductive cycle, than 

 the simple alternation of gemmation with sexual generation 

 under any of its modifications ; and the extent to which it is 

 carried is still more variable. Thus in the protomorphic al- 

 ternation of most Cestoid worms, we have but a single pri- 

 mary cyst, which directly originates the Tsenia-head ; but in 

 the Echinococcus there always is one, and frequently are se- 

 veral derivative cysts, interposed before the appearance of 

 the typical form. In the Trematoda again, we have two 

 intermediate forms as a general rule (the infusorial and the 

 gregariniforrn), but, according to Steenstrup, the latter is oc- 

 casionally repeated, and that perhaps more than once, be- 

 fore the appearance of the cercariform zooids.* In the 



* It is this variable multiplication of links which has been the source 

 of ambiguity as to the corresponding forms in the genetic cycle of the 

 Trematoda and Cestoidea respectively. Professor Van Beneden correctly 

 identifies the immediate product of the ovum in each case, under the 

 name of proscolex (the " infusorial embryo of the Distoma, and the 

 "six-hooked vesicle" of the Tcenia), but he is less satisfactory in his 

 identification (under the name of scolexj of the forms immediately suc- 

 ceeding that is, of the Redia of the Distoma and the Tgenia-head of the 

 Cysticercus because the length of the preliminary pullulation differs in 

 these species. If Filippi is correct in stating (Ann. Nat. Hist., 2d Ser., 

 XX., 130) that in some instances the primary infusorian zooid is itself 

 transformed into the redia, rather than that it generates the latter in its 

 interior, in this case the comparison would stand, for the infusorian and 

 Kedia are as much amalgamated as the " six-hooked vesicle" of the Tsenia- 

 egg is with the primary cyst of the Cysticercus. But the majority of the 

 species of Distoma, which have two links in their preliminary develop- 

 ment, should be compared, not with the Cysticercus, which has but one 

 the primary cyst budding off the Tsenia-head directly but with the 

 Echinococcus scolicipariensofKuchenmeiateT, in which the first cyst buds 

 off secondary cysts as the immediate progenitors of the heads. Those Dis- 

 tomata which have more than one Redia might in like manner be coir- 

 pared to the Echinococcus altricipariens of KUchenmeister, in which 



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