i STRUCTURE AND PHYSIOLOGY OF ANIMALS 41 



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

 of reproduction. Such a phenomenon is termed alternation of 

 generations or metagenesis. The term heterogamy is applied to 

 cases in which two different sexual generations usually a true 

 sexual and a parthenogenetic alternate with one another. 

 Pmdogenesis, 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. M. 1 W.- W. 3 buds in various stages of growth. (From 

 Parker's Biology.) 



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, AK) passing through the middle of the blas- 

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

 (ab, cd,) running at right angles to this to the outer surface. In a 

 symmetrical gastrula the secondary axes would be all equal. Many 



