68 Heredity. 



manifested any such power. It is impossible to believe 

 that Daplmia and the honey-bee have inherited from a 

 common parthenogenetic ancestor the power to produce 

 fertile unimpregnated eggs, for the one form is much 

 more closely related to normal insects and the other to 

 normal Crustacea than they are to each other. AVo 

 may therefore state with confidence that the power 

 has been independently acquired by many animals. 



In the second place, we must admit that partheno- 

 genetic ova are true ova in every sense : they are de- 

 veloped in an ovary like other eggs, and in many cases, 

 as in those butterflies which are occasionally partheno- 

 genetic, the very eggs which usually require impregna- 

 tion may in rare instances develop without it. Weis- 

 mann has made very careful examination as to the origin 

 of both kinds of eggs in Leptodora, a water-flea related to 

 Daphnia ("Ueber dieBildungvon "Wmtereier bei Lepto- 

 dora hyalina," Zeit. f. Wiss. ZooL, xxvii., 1876), and he 

 finds that while there is some difference in the mode of 

 origin of the winter eggs, which do not develop unless 

 they are fertilized, and the summer eggs, which are 

 parthenogenetic, the difference simply consists in the 

 amount of nourishment which they receive in the ovary. 

 In each case certain ova degenerate and are used up by 

 the others as food, and a winter egg thus absorbs a greater 

 number of these embryonic ova than a summer egg 

 does; but Weismann's observations show that each of 

 them is in all respects a true ovum, and that they are 

 perfectly homologous with each other. 



In some cases, as in some of the wasps described by 

 Bassett and Adler, the animal which is born, from a 

 parthenogenetic egg differs considerably in structure 

 from that which is born from a fertilized egg; but in 

 other cases, as in butterflies and moths, there is no such 



