ALTERNATION OF GENERATIONS. 115 



would seem that the Cercarise are not formed in the first 

 Gregarina-zooid at all, but in those of an intermediate 

 brood, derived from it, and of the same general character.* 



Till the recent researches of Professor Van Beneden of 

 Louvain, the whole series of changes had never been traced 

 out in the same species, or by one single observer, but, as 

 the author just quoted remarks in another place,^ this tes- 

 timony of different good and independent witnesses, at 

 different periods, to different stages of the successive genera- 

 tions, which, when compared with one another, are seen to 

 link together and complete the metagenetic cycle, was per- 

 haps the best foundation for scientific confidence in the 

 truly marvellous result. 



"Siebold observes the development and birth of the 

 ciliated monadiform embryo from the egg of the oviparous 

 Trematode Entozoon, and the escape of the gregariniform 

 non-ciliated worm from the ciliated one. 



" Bojanus and V. Baer trace the development of the 

 cercariform individuals from the 'King's yellow worm,' 

 which in its form and simple structure corresponds with the 

 vermiform offspring of the ciliated embryo. 



" Nitsch traces the Cercariform progeny of that worm to 

 their pupa state. 



" Steenstrup, confirming the origin of the Cercariae from 

 the multiparous ' King's yellow worm,' completes the obser- 

 vation of their metamorphosis through their pupa state, 

 into the Trematode Entozoon." 



In the development of the Trematoda, as exemplified in 

 the case of a Distoma, there are two points of especial inte- 

 rest in their bearing on the general question of Alternation : 



1. The imperfection and variability in the amount of 

 organization acquired by the zooids, which originate directly 

 from the infusorian form, as compared with that of the 



* Steenstrup on " Alternation," pp. 69-93. 

 f Parthenogenesis, p. 17. 



