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528 GENERATION. [CHAP. XXXVI. 



generation and those of the next succeeding, there may be several 

 series of these imperfect or larval forms; each larva producing 

 without any generative act, and, indeed, without itself possessing 

 true generative organs, many similar larval forms, until at last these 

 larvae, instead of producing larvae, give rise to perfect forms, which 

 propagate only by the production of ova. 



This curious phenomenon occurs amongst many classes of 

 animals ; and the subject of late years has engaged the attention of 

 many naturalists. Steenstrup has described the process under the 

 term alternation of generations. Owen terms it metagenesis and 

 parthenogenesis. The facts have been explained differently by dif- 

 ferent observers ; the two most important theories being the fol- 

 lowing : according to the first, the subsequent broods result by a 

 process resembling budding, taking place within the bodies of their 

 predecessors; while the second supposes that a portion of the 

 original germ-mass is actually transmitted from the parent through 

 the whole series of beings existing between two generative acts. 

 The latter view has been most ably advocated by Professor Owen, 

 in his lectures on Parthenogenesis ; and the former is supported by 

 Dr. Carpenter. In the Campanularia dichotoma, one of the tribe 

 of polyps, at certain periods, buds are developed from the stem, 

 which do not become converted into polyps, but, after having 

 reached a certain stage of development, drop off, and in their 

 mature state are seen as transparent disc- like bodies, having the 

 power of swimming about in the water. These creatures have 

 long been known as Medusae, or jelly-fishes. It must be remarked, 

 that no generative organs are to be found in the polyp ; but these 

 organs are found in the Medusae, in which also ova are developed. 

 The ova become polyps, which eventually put forth Medusa-buds 

 as before. 



In another polyp, the Strobila, at certain periods, multiplication 

 by the formation of buds ceases, and the body of the polyp be- 

 comes constricted, and at the same time much elongated. The 

 constrictions, which may be as many as forty in number, gradually 

 become deeper, until at length the body of the polyp becomes 

 divided into a number of flattened discs. The terminal disc drops 

 off, and appears as a free swimming Medusa, in which generative 

 organs are found and ova produced. The other discs fall off suc- 

 cessively, and in like manner become Medusae. These polyps, 

 therefore, would with propriety be considered as belonging to the 

 class Acalephae, the Medusa representing the perfect condition of 

 these animals. 



