STRUCTURE AND PHYSIOLOGY OP ANIMALS 



41 



alternates with a stage in which there occurs a true sexual mode 

 ot reproduction. Such a phenomenon is termed alternation of 

 generations or meta;/c)ied><. The term hrtcrogamy is applied to 

 cases m which two different sexual generations— usually a true 

 sexual and a parthenogenetic— alternate with one another. 

 PrnJogencsis, or the development of young by a sexual process from 



Fig. 27.— Fresh-water polype (Hydra), two specimens, the one expanded, the other contracted, 

 showing multiplication by budding, ftti.l it/.i^ 6(^.3 buds in various stages of gi-owth. (Fi'oni 

 Parker's Biologii.) 



individuals that have not attained the adult condition, is a 

 phenomenon which is to be observed in some groups of animals. 



7. Symmetry. 



The general disposition or symmetry of the parts in an animal 

 presents two main modifications — the radial and the bilateral. 

 The gastrula (p. 23) is the simplest and most generalised form among 

 multicellular animals or Metazoa ; but no adult animal retains 

 this simple shape. In the gastrula we may imagine a central 

 primary axis (Fig. 28, AB) passing through the middle of the blas- 

 topore and of the archenteric cavity, and a series of secondary axes 

 {ah, cd,) running at right angles to this to the outer surface. In a 

 symmetrical gastrula the secondary axes would be all equal. Many 



