REPRODUCTION AND DEVELOPMENT. 



275 



larger 



the zoophytic form, which is the more largely developed 

 condition, gives origin by gemmation to medusoids bearing 

 male and female elements, and from the fertilised ova of these 

 medusoids new colonies of zoophytes take origin. There are 

 many other examples of alternation of generation among 

 animals; but the interest in this case is increased by the 

 circumstance, that in other hydroid zoophytes the buds which 

 bear the sexual elements remain attached as mere organs of 

 the zoophyte, never attaining to an independent existence; and 

 thus one is enabled to see that the power of the medusoid to 

 reproduce the zoophyte is but a modification of the more fre- 

 quently exemplified power of a part to reproduce a whole , 

 the modification being that the reproductive part is entirely 

 severed before the sexes are developed. In the 

 Medusae, or jelly fish, a similar 

 alternation takes place, only the 

 sexual form is the more largely 

 developed; the ovum takes root 

 and grows into a body which 

 breaks up into a series of discs, 

 each of which is developed into 

 a medusa. 



In the Aphides, or plantlice, 

 another cycle is exhibited. In 

 the interior of the individuals 

 derived from fertilized ova, 

 another generation is developed V / 

 from unimpregiiated germs ; and \ . / 

 the insects so formed become J\A 

 parents of others in like manner; j$ 

 and only after several genera- 

 tions are perfectly sexual indi- Fig> 533. ^ALTERNATE GENE- 



viduals produced, with whom - 

 the cycle again begins. 



In bees there occurs a descrip- 

 tion of true parthenogenesis. 

 The queen, in her marriage 

 flight at the time of swarming, 

 receives the male element into a 

 sac provided for the purpose, and afterwards in laying her 



RATION of Aurelia aurita. A, 

 Ciliated ovum become ad- 

 herent. B, The same at a 

 later stage sending out pro- 

 cesses. C, Strobilus, which 

 breaks up into separate cups, 

 each of which becomes an 

 adult Aurelia. After Sars. 



