EMBEYONIC FISSION IN CyOlOSTOMATOUS POLYZOA. 33 



young developmental stages." A similar suggestion with 

 regard to the origin of alternation of generations among the 

 Hydromedusse has also been elaborated by Brooks (6), who 

 supposes that the hydroid stage has been evolved by the ac- 

 quirement of the power of budding by the fixed larval stage. 



A slight modification of the "primary embryo" of Crisia 

 would suffice to make it necessary to consider the life-history 

 of that animal as a case of alternation of generations. But 

 since, as I believe, the budding structure consists of a mass of 

 embryonic cells, which \iltimately becomes completely con- 

 verted into " secondary embryos," leaving nothing behind, I 

 have preferred not to describe it as a separate generation. 



Enough has been said to show that in the Tunicata at least, 

 and to a less extent in the Coelenterata,^ there are remarkable 

 cases of the formation of buds from slightly differentiated 

 masses of cells. These two groups, with the Polyzoa, are 

 certainly the groups' of animals in which budding in the adult 

 condition is a more normal event than in other groups of 

 animals. 



It may thus be asserted that in the Polyzoa, the Tunicata, 

 and the Coelenterata the asexual reproduction of certain forms 

 takes place at a stage before the individual which is reproduc- 

 ing asexually has had time to undergo more than the earliest 

 steps in its development. A similar precocious formation of 

 fresh individuals is well known in the reproduction of 

 Trematoda.^ 



The investigations of Driesch (9) and of Haeckel (13) have 

 shown that blastomeres which have been artificially separated 

 from the embryo are able, in some cases, to give rise to com- 

 plete larvae. The question suggests itself: Has the gemmi- 



' The case of Cunina, as described by Uljanin, Schuize, MetschnikofF, and 

 Brooks, and more recently by O. Maas ('Zoolog. Jahrbiiclier,' " Abtli. f. 

 Anat. u. Ontog.," Bd. v, Heft 3, 1892), is anotiier remarkable instance of the 

 same kind. 



* Compare particularly the remarkable account given by Heckert (18) of 

 the life-history of Distoma macrostomum, and particularly the state- 

 ments referring to its remarkable branched sporocyst, known as Leuco- 

 chloridium paradoxum. 



VOL. VI. 3 



