REPRODUCTION: ALTERNATION OF GENERATIONS 279 



where they float around until the eggs and the sperms come 

 in contact and fuse, producing a typical fertilization. The 

 jellyfish itself, after it has extruded the sex bodies, has no 

 further function, and dies. The 

 egg, however, now grows into 

 a new colony like the origi- 

 nal. This jellyfish is evi- 

 dently the sexual stage in the 

 development of the hydroid, 

 and corresponds to the sexual 

 stage in the development of 

 the fern (the prothallium). 



The alternation of a sexual 

 with a non-sexual method is 

 far more common among 

 plants than among animals. 

 It is developed in all plants 

 except the lower orders, even 

 the flowering plants, as we 

 have just seen, having such an 

 alternation. Among animals, 

 however, alternation of genera- 

 tions is found only in the lower orders. It is common among 

 the Hydr&ids, and a modified form of it occurs in one of the 

 higher animals (Salpa)', but among the great majority of ani- 

 mals, when sexual reproduction is developed, the non-sexual 

 method is totally lost. 



FIG. 130. A FULL-GROWN 



JELLYFISH 

 m, mouth; g, gonads. 



