28 MANUAL OF ZOOLOGY. 



ness is extremely striking. The ovarian bud, however, differs 

 from the true gemm» or buds in its inability to develop itself 

 into an independent organism, unless previously brought into 

 contact with another special generative element. The only 

 exceptions to this statement are in the rare cases of true " par- 

 thenogenesis," to be subsequently alluded to. 



b. Reproduction by Internal Gemmation. — Before considering 

 the phenomena of " alternate generations," it will be as well to 

 glance for a moment at a peculiar form of gemmation exhi- 

 bited by some of the Polyzoa, which is in some respects in- 

 termediate between ordinary discontinuous gemmation and 

 alternation of generations. These organisms are nearly allied 

 to the sea-mat, already spoken of, and, like it, can reproduce 

 themselves by continuous gemmation (forming colonies), by a 

 true sexual process, and rarely by fission. In addition to all 

 these methods they can :ftproduce themselves by the formation 

 of peculiar internal buds, which are called "statoblasts." These 

 buds are developed upon a peculiar cord, which crosses the 

 body-cavity, and is attached at one end to the fundus of the 

 stomach. When mature they drop off from this cord, and. lie 

 loose in the cavity of the body, whence they are liberated on 

 the death of the parent organism. When thus liberated, the 

 statoblast, after a longer or shorter period, ruptures and gives 

 exit to a young Polyzoon, which has essentially the same 

 structure as the adult. It is, however, simple, and has to 

 undergo a process of continuous gemmation 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, inde- 

 I)endently 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 gemmce 

 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 gemmation ; 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 pheno- 

 mena, 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 per- 



