REPRODUCTION. 35 



rise to fresh Hydrse by a true generative process. It must 

 also be borne in mind that this production of fresh zooids by a 

 process of gemmation is not so essentially different from the true 

 sexual process of reproduction as might at first sight appear, 

 since the ovum itself may be regarded merely as a highly 

 specialised bud. In the Hydra, in fact, where the ovum is pro- 

 duced as an external process of the wall of the body, this like- 

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

 from the true gemmae 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 pheno- 

 mena of " alternate generations," it will be as well to glance for a moment 

 at a peculiar form of gemmation exhibited by some of the Polyzoa, which 

 is in some respects intermediate between ordinaiy discontinuous gemma- 

 tion 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 reproduce 

 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 invariable absence 

 of germinal vesicle and germinal spot, and their never exhibiting the phe- 

 nomena of yelk-cleavage, independently 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 gemnuz pecu- 

 liarly encysted, and destined to remain for a period in a qwiescent 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 pro- 

 ducing 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 sexuaj organs, and is capable of performing no 

 other function than that of nutrition, whilst the other is pro- 



