ALTERNATION OF GENERATIONS 



179 



This is the case in the alternation of generations in medusa;. 

 The polype is the original form, and even at the present day the 

 fertilised ovum of the medusa gives rise to a polype in most 

 species. By the budding of this polype, or at any rate of the 

 offspring which have been produced by gemmation, medusae are 

 again developed. If, for the sake of simplicity, we neglect the 

 slight differences which may exist between the germ-plasm of 



Fig. -lo.— BougainviUea raiiiosa. (After AUman.) Polype stock with gastro- 

 zooids (A) and medusoid buds (;«/t) ; in, young Medusa {Margelis raiiiosa), 

 which has become free. (From A. Lang's ' Lehrbuch der vergleichenden 

 Anatomic.') 



the egg and that of the bud. it is evident that two germ-plasms 

 take part in tlie cycle of development of the species, and these 

 must differ as regards very many, if not almost all, of their deter- 

 minants, for the medusa is jMovided with a number of parts and 

 organs which the simple polype does not possess. Thus we 

 must assume that there are two different kinds of ids ofwliich 



