PULLULATIOX IN THE GENETIC CYCLE. 11-7 



meet contingencies which would otherwise imperil the con- 

 tinuance of the race. Thus in the Aphides the male and 

 female insects appear at the close of the season, when it is 

 necessary for the preservation of the species that true ova 

 should he formed, more capable of resisting the winter cold 

 than the rudimentary forms, which are propagated from eacli 

 other in such quick succession during the summer. 



§ 2. Such a process of continuous gemmation may 

 occur, as it would seem, at any period in the life-history of 

 a species. We have an instance of it during the course of 

 germinal development in Distoma and Echinococciis among 

 animals, and in the successive forms of the capsule and 

 protonema of moss-plants ; and we meet with it also 

 at the other extreme of the cycle, in the medusoid offsets of 

 the Polypifera. Professor E. Forbes makes mention of four 

 modes of gemmation among the Medusae : — 1. From the 

 ovaries, as noticed by Sars in Thaumantias ; 2. from the 

 peduncular stomach [manubrium] in Lizzia \^Cyta:'is\, the 

 gemmae coming off from each of its four sides in a some- 

 what symmetrical way ; 3. from the walls of a tuljulav 

 proboscis into which the manubrium is extended, in a 

 species of Sarsia, the gemmai coming off in an irregular 

 manner from its whole lenoth ; and 4. from the bases or 

 tubercles of the four marginal tentacles, in another species 

 of the same genus.* 



* Naked-eyed Medusae (Ray Soc), pp. 17, 65, 59, 58. See also En<^- 

 lish Cyclop. Nat. Hist., I., 25, and Carpenter's Principles of Comp. 

 Physiol., 4th Ed., p. 558. 



Dr. T. S. Wright, indeed, regards every medusa as composite, looking 

 on its component memloers as so many modified polypes. He terms the 

 oral 'protubei'ance an alimentary polype, and the contractile marginal 

 filaments and the ovaries, tentacular and sexual j^olji^es, comparing them 

 to the parts fulfilling the same functions in Hydractinia — of whose ge- 

 neral polype nature there can be no doubt (Ed. N. Phil. Jour., 1857, 

 pp. 315 and seq.) Huxley again regards the whole medusa as a modified 

 polype, and he demonstrates very clearlj' its agreement in the general 

 plan of structure. The truth seems to be that in proportion as the diffuse 



n 2 



