ALTERNATION OF GENERATIONS. 1 1 1 



coiiGerned in which the budding process stands to the sexual 

 act, and to the full development of the specific type — rela- 

 tions depending on the period of the life-history of the 

 species, at which the act of gemmation is interpolated in 

 the genetic cycle. The contrast lies especially between the 

 cases in which the alternation of form is due to zooids 

 being budded off in the Protomoiyhic stage of the life- 

 history — that is, during the early progTCss of germinal de- 

 velopment — and those in which it arises from the detach- 

 ment of gemmae in the fully developed or typical phase, as 

 a preliminary step to the evolution of reproductive organs 

 — the latter zooids belonging to the Gamomorphlc stage, or 

 that of sexual maturation. The two classes — as has been 

 already observed — differ wddely in their structure and rela- 

 tions. In the one case they are the primary products of 

 impregnation, precursors of the perfect form, and without 

 sexual characters — in the other derivative, and wiWi dis- 

 tinct sex. Zooids of both kinds, indeed, may have cer- 

 tain organs superadded, varying in their nature and com- 

 pleteness with the circumstances of their life as indepen- 

 dent beings. In those of the protomorphic stage, the ad- 

 ventitious organization probably does not go beyond the 

 development, externally, of cilia, or of a contractile integu- 

 ment for locomotion, and internally, of a rudimentary di- 

 gestive apparatus ; but in many gamomorphic zooids, both 

 the locomotive and alimentary systems may be rather 

 highly organized, and the whole structure occasionally 

 larger and more complex and elaborate than that of the 

 parent stock. On the other hand, such is the structural 

 degradation of some zooids of both kinds, that they might 

 readily pass for mere proliferous cysts or egg-sacs. This 

 variability in the kind and extent of organization proves 

 of itself its adventitious nature, and shows it to be of no 

 value as a distinctive feature. The real points of distinc- 

 tion are those before referred to — their position in the 



