ALTERNATION OF GENERATIONS 



175 



It will not be unin- 

 teresting to give some 

 examples by way of 

 illustration. 



We will first take a 

 case of heterogenyj or 

 alternation of gener- 

 ations in which the 

 two generations do not 

 differ at all from one 

 another in the full- 

 grown condition. As 

 a rule, the difference 

 between the two suc- 

 cessive generations in 

 the DapJmidcE or water- 

 fleas, for example, con- 

 sists in the fact that 

 one generation is de- 

 veloped from summer- 

 eggs, w^hich contain a 

 small amount of yolk, 

 while the other arises 

 from winter-eggs, in 

 which the yolk is very 

 abundant. From both 

 of these two kinds of 

 eggs similar females 

 are developed : — the 

 complication arising 

 from the periodic ap- 

 pearance of the male 

 may be neglected for 

 the present. The sum- 

 mer-eggs are nour- 

 ished by the blood of 

 the mother, while the 

 winter-eggs are not ; for 

 the amount of yolk in 

 the latter necessitates 

 a different kind of 



Fig. 8. — A female o^ Daphnia pulex, 

 with two parthenogenetic eggs in 

 the brood-chamber {J)). (After R. 

 Hertwig.) 



