ALTERNATION OF GENERATIONS. 55 



unisexual condition of permanent maleness or femaleness is 

 a secondary differentiation. The cases which we have cited 

 above may be interpreted as due to persistence of the primi- 

 tive condition, or as reversions to it. 



{b) Parthenogenesis, as we know it, is a degenerate form 

 of sexual reproduction, in which ova produced by female 

 organisms develop without being fertilised by male elements. 

 It is well illustrated by Rotifers, in which fertilisation is not 

 known to occur, while in some genera males have never 

 been found ; by many small Crustaceans whose males are 

 absent for a season ; by aphides, from among which males 

 may be absent for the summer (or in artificial conditions for 

 several years) without affecting the rapid succession of female 



'2£o_;> r'r? 



Fig. 6. — Diagrammatic expression of alternation of 

 generations. 



I. Hydi'omedusas. 



cz'. Fertilised ovum gives rise to asexual form ^,which, by budding, 

 produces sexual form or forms .S"; in case of Hydromedusae A is re- 

 presented by hydroid (//) and .S' by medusoid (M). 



II. Liver Fluke. 



oz>. Fertilised ovum gives rise to asexual stages (.'I) which, from 

 special spore-like cells (/O ), produce eventually the sexual fluke (S). 



generations ; by the production of drones in the bee hive, — 

 for the eggs which give rise to drones are unfertilised {see 

 p. 60). 



(c) Alternation of Generations. — A fixed asexual hydroid 

 or zoophyte (campanularian or tubularian) often buds off 

 and liberates sexual medusoids or swimming bells, whose 

 fertilised ova develop into embryos which become fixed and 

 grow into hydroids (Fig. 49, p. 156). This is the simplest 

 illustration of alternation of generations, which may be 



