PULLULATION IN THE GENETIC CYCLE. 153 



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

 dental nature of the distinction. 



§ 5. The repetition of many successive gemmations — or 

 what has been here termed pullulation — must be considert-tl 

 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 Ttenia-head ; but in 

 the Echinococciis 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 

 gregariniform), 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 mixltiplication of links wliicli lias been the sonrce 

 of ambiguity as to the con-esponding forms in the genetic cj'cle 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 ToeniaJ, but he is less satisfactory in his 

 identification (under the name of scolex) of the forms immediately suc- 

 ceeding — that is, of the Redia of the Distoma and the Taeuia-head of the 

 Cysticercus — because the length of the preliminary pullulation difiers 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 

 ti'unsformed into the redia, rather than that it generates the latter in its 

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

 Redia are as much amalgamated as the " six-hooked vesicle" of the Ttenia- 

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

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

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

 — the primary cyst budding off the Taenia-head directly — but with the 

 Echinococcus scolicvpariens of Kuchenmeister, 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 con - 

 pared to the Echinococcxis oJtricipariens of Kuchenmeiator, in which 



H 3 



