460 



ZOOPHYTES. 



Sexual process ; the fecundating apparatus, or vesicle-producing 

 "sperm-cells," and the ovum (containing the "germ-cell," im- 

 bedded in a store of nutriment adapted for its eai-ly development), 

 being evolved in the substance of the walls of the stomach, the 

 former just beneath the arms, the latter nearer to the lower end 

 of the body. It would appear that sometimes one individual 

 Hydra developes only the male cysts or sperm-cells, while another 

 developes only the female cysts or ovisacs ; but the general rule 

 seems to be, that the same individual forms both organs. The 

 fertilization of the ova, however, cannot take place until after 

 the rupture of the spermatic cyst and that of the ovisac also ; so 

 that the parent has no further participation in it, than has the 

 Fucus in the analogous fertilization of its germ-cells after their 

 discharge (§ 205). Although the production, from such an egg, 

 of a new Hydra, similar in all respects to its parent, has not yet 

 been witnessed, there seems no reason to doubt the fact. It 

 would seem that this alternation in the method of reproduction, 

 between the gemmiparous and the sexual, is greatly influenced 



FlQ. 221. 



Fig. 222. 



Fig. 221. Rydrafusca in gemmntion ; a, itioulh | b, base ; c, origin of one of tlie Irads. 



Fig. 222. Development of Medusa-buds in Symmjna Sarsii:—k, an ordinary polype.'wilh its club- 

 shaped body covered wilh tenlacula:— b, a polype putting forth medusan gemma); a, a very young 

 bud; 6, a bud more advanced, the quadrangular form of which, with the four nuclei whence the 

 cirrhi afterwards spring, is shown at d; c, a bud still more advanced. 



by external temperature ; the eggs being produced at the ap- 

 proach of winter, and serving to regenerate the species in the 



