IN THE EMBEYOGENY OF THE HIGHER ANIMALS. 161 



nor those which afterwards pullulate from them, ever be- 

 come quite detached ; the whole cohering together to form 

 a compound animal while the original germinal matrix 

 becomes as completely reduced to the condition of a mere 

 appendage of the structures derived from it, as in the case 

 of any vertebrate ovum. 



5. For another illustration of a reproductive process, 

 intermediate between the continuous embryogeny, as it may 

 be termed, of the higher species, and " alternation of gen- 

 erations," we may turn to the Cystic Entozoa, which, prior 

 to their most notable change the transformation into 

 Cestoid worms present us with a series of successive 

 forms, all referable to the Cystic phase. The typical form 

 the Tsenia-head is not, as has been shown, produced at 

 once from the egg, but is budded off as an aftergrowth from 

 the cyst which is the primary development. With consider- 

 able differences of detail, this general relation is to be 

 traced in all the species. In Echinococcus the Tsenia-heads 

 are developed in large numbers, by a sort of gemmation, from 

 the lining membrane either of the primary cyst, or of others 

 derived from it by an intervening process of pullulation. 

 In Coenurus also there is a formation of numerous heads 

 from the original cyst ; only they are budded off from a 

 special thickening of the membrane, not from the whole 

 interior, and they subsequently become evaginated so as to 

 protrude from its exterior. In both these cases there is an 

 obvious " alternation," attended with a multiplication of 

 the brood, but in Cysticercus the germ-mass is resolved into 

 a simple cyst, from which is evolved a single Tsenia-head, 

 the whole process having somewhat the character of a con- 

 tinuous development, like the formation of the embryo of a 

 vertebrated animal.* Indeed, not only is the latter formed 



* A detailed account of the primary development of the vesicle and 

 head of the Cysticercus is given by Mr. Rainey in the Philosoph. Transact, 

 for 1857. 



