STRUCTURE AND PHYSIOLOGY OF ANIMALS 



41 



6. THE REPRODUCTION OF ANIMALS. 



In a limited number of groups of animals reproduction takes 

 place by means of cells corresponding to ova developed in organs 

 similar to ovaries, but without impregnation by means of sperms. 

 This phenomenon is known as parthenogenesis (cf. p. 21). 



Besides the sexual process of reproduction by means of ova and 

 spermatozoa, there are in many classes of animals various asexual 

 modes of multiplication. One of these the process of simple binary 

 fission has been already noticed in 

 connection with the reproduction of 

 Amoeba. The formation of spores 

 by multiple fission is an asexual mode 

 of multiplication which occurs only 

 in the Protozoa, and will be described 

 the account of that group. 



in 



Multiplication by budding takes 

 place in a number of different 

 classes of animals. In this form of 

 reproduction a process or bud (Fig. 

 27, bd) is given off from some part 

 of the parent animal ; this bud 

 sooner or later assumes the form of 

 the complete animal, and may 

 become detached from the parent 

 either before or after its develop- 

 ment has been completed, or may 

 remain in permanent vital con- 

 nection with the parent form. 



When the buds, after becoming 

 fully developed, remain in vital FIG . 2 -._ i;i , sh , vaterpoIype(Hydra) ; 



continuity with the parent, a sort two specimens, the one expanded, the 



of compound animal, consisting 



of a greater or smaller number 



of connected units, is the result. 



Such a compound organism is termed a colony, and the component 



units are termed zooids. In some cases such a colony is produced 



by a process which is more correctly termed incomplete fission 



than budding. 



Alternation of Generations ; Heterogamy ; Psedogenesis.- 

 In the life-history of a considerable number of animals, a stage in 

 which reproduction takes place by a process of budding or fission 

 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. 



>ther contracted, showing miiltiplica- 

 tion by budding, bd.^bd. 2 M. : - bmls 

 in various stages of growth, (l^rom 

 Parker's fiiology.) 



