274 ANIMAL PHYSIOLOGY. 



by mere gemmiparous reproduction, sexual reproduction is 

 always resorted to after a cycle has been passed through. 



The modes in which such cycles are accomplished are very 

 various. In many instances, both in cryptogamic plants and 

 in the lower forms of animals, there is a manifest alternation 

 of generation. Thus the spores on the frond of a fern are 



Fig. 137. ALTERNATE GENERATION OF HYDROID ZOOPHYTE (Bougain- 

 villia ramosa). A, Zoophytic form ; a t medusoid. B, Liberated 

 medusoid. After Allman. 



asexual germs, which grow up to form a lichen-like plant, 

 the prothallus; the prothallus bears upon it male and femalo 

 elements, and from the union of these the young fern takes 

 its rise. So also among animals, in many hydroid zoophytes 



