104 ELEMENTS OF BIOLOGY. 



before it can assume the compound form proper to the 

 adult. 



As regards the nature of these singular bodies, " the in- 

 variable absence of germinal vesicle and germinal spot, and 

 their never exhibiting the phenomena of yelk-cleavage, in- 

 dependently of the conclusive fact that true ova and ovary 

 occur elsewhere in the same individual, are quite decisive 

 against their being eggs. We must then look upon them 

 as gemma peculiarly encysted, and destined to remain for a 

 period in a quiescent or pupa-like state." (Allman). 



c. Alternation of Generations. In the case of the Hydra 

 and the sea-mat, which we have considered above, fresh 

 zooids are produced by a primordial organism by gemma- 

 tion ; the beings thus produced (as well as the parent) being 

 capable not only of repeating the gemmiparous process, but 

 also of producing new individuals by a true generative act. 

 We have now to consider a much more complex series of 

 phenomena, in which the organism which is developed from 

 the primitive ovum produces by gemmation two sets of 

 zooids, one of which is destitute of sexual organs, and is 

 capable of performing no other function than that of nutri- 

 tion, whilst the other is provided with reproductive organs, 

 and is destined for the perpetuation of the species. In the 

 former case the produced zooids all resembled each other, 

 and the parent organism which gave rise to them ; in the 

 latter case, the produced zooids are often utterly unlike 

 each other and unlike the parent, since their functions are 

 entirely different. 



The simplest form of the process is seen in certain of the 

 Hydroid Zoophytes, such as Hydractinia (fig. 29). The 

 embryo of Hydractinia emerges from the egg as a free- 

 swimming ciliated body, which, after a short locomotive 

 existence, attaches itself to some marine object, develops a 

 mouth and tentacles, and commences to produce a colony 

 of zooids like itself, by a process of continuous gemmation. 

 The zooids thus produced remain permanently in connec- 



