SEX INHERITANCE 97 



living male it must be assumed that the no-X 

 sperm is non-functional. The X-bearing sperms 

 would fertilize the eggs and produce the hermaphro- 

 ditic females. 



Parthenogenesis and Sex 



In the animal kingdom there are several cases 

 where a species reproduces for several generations 

 by parthenogenesis, and, then, at the end of the 

 chain, sexual forms appear. The rotifer, Hydatina 

 senta, is the best illustration since the change can 

 now be controlled (Whitne}^- This animal ordi- 

 narily produces parthenogenetic eggs (Fig. 33, 

 A, D), which give rise to females. If fed on a pure 

 diet of the protozoon, Polytoma, only these par- 

 thenogenetic females are produced in successive 

 generations (Whitney, Shull) . If the diet is changed 

 to the green alga (Euglena) a female will tlien lay 

 eggs that give rise to a new kind of individual — a 

 male-producing female whicli is not externally 

 different from its mother. If the male-producing 

 female is not fertilized by a male, soon after hatch- 

 ing (B), she produces a large number of small eggs 

 (E) that develop partlionogenetically into males 

 (C), but if she is fertihzod she produces a few large 

 eggs with thick shells (F)— the resting eggs— that 

 always become females. Hence the cliangt^ in diet 

 has caused the appearance of a new kind of in- 

 dividual that functions as a sexual femak^ witli. the 

 production of a few daughters, or as a partheno- 



