develop:\ient. loi 



4. Relation between zooidal and embryonal multiplication in the Htdroida ; Polymerism 

 and Ileteromorphism ; Genetic succession of zooids. 



Having now examined tlie various modes, whether zooidal or embryonal, by which multipli- 

 cation is effected in the IIydroida, it remains for us to see how the two forms of reproduction 

 are related to one another, and how they are associated in the complex phenomenon which con- 

 stitutes the life of the hydroid. 



From all the facts which the study of the IIydroida has made apparent, we may regard it 

 as certain that however long zooidal multiplication may continue, this is not sufficient for the 

 perpetuation of the species, but that a period must at last come in the life of the hydroid when, 

 by an act of true sexual reproduction, new individuals are produced for the indefinite extension 

 of the species through time. 



These facts find their expression in a remarkable law originally propounded by Chamisso, 

 when he made his memorable discoveiy of the true genetic relation between the solitary Salpa 

 and the associated chain-like colonies of these animals ; though it was reserved for Steenstrup, by 

 correlating with Chamisso's discovery, not only the genetic phenomenon of the IIydroida, but also 

 various analogous phenomena observed in other members of the animal kingdom, to give a wider 

 comprehensiveness and a more definite enunciation to the law henceforth known as the law of 

 "alternation of generations," an expression originally employed by Chamisso himself when describing 

 the genetic phenomena of Salpa. It is true that Steenstrup's mode of stating his law of alternation 

 of generations was destined to undergo some modification, but it has, nevertheless, received in 

 all essential points abundant confirmation, and will explain, in a way which it alone can do, a 

 host of phenomena which would otherwise have appeared isolated and exceptional.' 



The law of alternation of generations manifests itself wherever it prevails, in the fact that 

 every act of embryonal development is followed by one or more acts of zooidal development, 

 which invariably conduct us to an ovum in which embryonal development, followed by zooidal 

 development, again occurs, and the entire series becomes thus repeated. 



Now, the various series expressing this alternation of sexual with non-sexual development 

 exhibit among the Hydroida different degrees of complication, which will be more easily under- 

 stood if we attempt to present them in the somewhat technical shape of formulae. 



Let t be the trophosomc, and y the gonosome then — 



I. < + /7 X ^H-.y X '+,^ X • • ■ &c. 



will be the general expression for the genetic succession in the life of the hydroid, the sign +, 

 indicating succession by zooidal development, and X by embryonal. 



' The true significance of the facts on which the law of Alternation of Generations is founded, 

 was for tlie first time clearly pointed out by Dr. Carpenter. See ' Brit, and For. Med.-Chir. Rev.,' 

 vol. i, Jan., 1848, p. 183, &c. 



