MODES OF SEXUAL REPRODUCTION. 



57 



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 

 or zoophyte often buds off and liberates sexual medusoids 

 or swimming-bells, whose 

 fertilised ova develop in- 

 to embryos which become 

 fixed and grow into hydroids 

 (Fig. 71, p. 150). 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 (Distomum 



\ hepaticuni) of the sheep 

 produces eggs which, when 

 fertilised, grow into embryos. 

 Within the latter, certain 

 cells (which might be called 

 spores) grow into numerous 

 other larvae of a different 

 form. Within these the 

 same process is repeated, 



and finally the larvae thus produced grow (in certain con- 

 ditions) into sexual flukes (Fig. 98, p. 189). In this case, 

 reproduction by special cells, like undifferentiated precocious 

 ova, alternates with reproduction by ordinary fertilised egg- 

 cells. So, too, the vegetative sexless " fern-plant " gives rise 

 to special spore cells, which develop into an inconspicuous 

 bisexual " prothallus," from the fertilised egg-cell of which 

 a " fern-plant " springs. 



Various kinds of alternation are seen in the life-cycle of 

 the fresh-water sponge, in the stages of the jelly-fish Aurelia^ 



FIG. 26. Diagrammatic expression 

 of alternation of generations. 



1. Hydrpmedusae. 



ov. Fertilised ovum gives rise to an 

 asexual form A, which, by bud- 

 ding, produces sexual form or 

 forms iS" ; in the case of Hydro- 

 medusae, A is represented by 

 hydroid (//), and 6" by medu- 

 < soid(Af). 



2. Liver Fluke. 



ov. Fertilised ovum gives rise to 

 asexual stages (A), which from 

 special spore-like cells (/?) pro- 

 duce eventually the sexual 

 fluke (S). 



