ALTERNATION OF GENERATIONS 



6s 



organism 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 conditions for several 

 years) without affecting the rapid succession of female 

 generations ; by the production of drones in the bee-hive 

 from eggs which are never fertilised. 



{c) Alternation of generations. — A fixed asexual hydroid 



I. 



^-^ 



^R rr;^ /^r r r^ 



— ^ 



Fig. 30. — Diagrammatic expression of alternation 

 of generations. 



1. HydromedusaB. 



ov., Fertilised ovum (ov.) gives rise to an asexual form A, 

 which, by budding, produces sexual form or forms 

 S ; in the case of Hydromedusae, A is represented 

 by hydroid (H), and S by medusoid (M). 



2. Liver Fluke. 



ov., Fertilised ovum (ov.) gives rise to asexual stages (A), 

 which, from special spore-like cells (R), produce 

 eventually the sexual fluke (S). 



or zoophyte often buds off and liberates sexual medusoids 

 or swimming-bells, whose fertilised ova develop into 

 embryos which become fixed and grow into hydroids 

 (Figs. 90 and 107). 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. 30). 



The liver-fluke (Distomum hepaticum) of the sheep 

 produces eggs which, when fertilised, grow into embryos. 

 Within the latter, certain cells (which might be called 



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