GENESIS. 213 



presented by these groups, arising from the fact that the 

 Buccessive generations of sexless individuals produced by 

 budding, are in some cases continuously developed, and in 

 others discontinuously developed ; and from the fact that, in 

 some cases, the sexual individuals give off their fertilized 

 germs while still growdng on the parent-polypedom, but in 

 other cases, not until after leaving the parent-polypedom and 

 undergoing further development. Where, as in all 



the foregoing kinds of agamogenesis, the new individuals 

 bud-out, not from any specialized reproductive organs, but 

 from unspecialized parts of the parent ; the process has been 

 named, by Prof. Owen, metagenesis. In most instances, the 

 individuals thus produced, grow from the outsides of the 

 parents — the metagenesis is external. But there is also a 

 kind of metagenesis which we may distinguish as internal. 

 Certain entozoa of the genus Didoma, exhibit it. From the 

 ^^^ of a Distoma, there results a rudely-formed creature 

 known to naturalists as the ^' King's-yellow worm.'' Gradu- 

 ally as this increases in size, the greater part of its inner 

 substance is transformed into young animals called Cercarim 

 (which are the larvae of Distomata) ; until at length, it 

 becomes little more than a living sac, full of living offspring. 

 In the Distoma pacijica^ the brood of young animals thus 

 arising by internal gemmation, are not CercaricE^ but are of 

 the same form as their parent : themselves becoming the 

 producers of Cercarice after the same manner, at a subsequent 

 period. So that sometimes the succession of forms is repre- 

 sented by the series A, B, A, B, &c. ; and sometimes by the 

 series A, B, B, A, B, B, &c. Both cases, however, exemplify 

 internal metagenesis, in contrast with the several kinds of 

 external metagenesis described above. That agamo- 



genesis which is carried on in a reproductive organ — either 

 a true ovarium or the homologue of one — has been called, by 

 Prof. Owen, parthenogenesis. In his work published under 

 this title^ he embraced those cases in which the buds arising 

 in the pseud- ovarium, are not ova in the full sense of the 



