MODES OF SEXUAL REPRODUCTION. 



57 



hermaphroditism is a primitive condition, and that the 

 unisexual condition of permanent maleness or female- 

 ness is a secondary differentiation. Other facts, such 

 as the hermaphroditism of many parasites, where cross- 

 fertilisation would be difficult, suggest that the bisexual 

 condition may have arisen as a secondary adaptation. It 

 seems likely that there is both primitive and secondary 

 hermaphroditism. 



(/>) 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 the 

 exception (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 con- 

 ditions for several years) 

 without affecting the rapid 

 succession of female genera- 

 tions; by the production 

 of drones in the bee-hive, 

 from eggs which are never 

 fertilised. 



(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. 77, p. 163). This is the 

 simplest illustration of alternation of generations, which 

 may be defined as the alternate occurrence in one life- 

 cycle of two (or more) different forms differently produced 

 (Fig. 26). 



The liver-fluke (pistomumhepaticuiri)Qi\he sheep produces 



FIG. 26. Diagrammatic expression 

 of alternation of generations. 



1. Hyclromedusae. 



Oc'. Fertilised ovum gives rise to an 

 asexual form A, which, by bud- 

 ding, produces sexual form or 

 forms S ; in the case of Hydro- 

 medusa-, A is represented by 

 hydroid (//), and 5 by medu- 

 soid (J7). 



2. Liver Fluke. 



oz>. Fertilised ovum gives rise to 

 asexual stages (A), which, from 

 special spore-like cells (A"), pro- 

 duce eventually the sexual 

 fluke (.S). 



