APOMIXIS AND RELATED PHENOMENA 413 



Parthenogenesis and Heteroploidy. — Although heteroploidy occurs 

 much less commonly in animals than in plants, there is some evidence 

 that here also a causal relationship often exists between apomixis, 

 heteroploidy, and hybridization. In Tephrosia the crossing of two non- 

 parthenogenetic species resulted in intermediate hybrids which produced 

 parthenogenetic eggs. A race of Daphnia, in which the egg forms only 

 one polar body without haplosis and develops parthenogenetically, is 

 evidently hexaploid. In Trichoniscus provisorius there is a diploid race 

 with normal sexual reproduction and a triploid race showing unreduced 

 parthenogenesis. ^'* 



As in the case of plants (p. 407), one may arrange a series of instances 

 in animals illustrating a transition from normal meiosis with sexuality 

 to the absence of meiosis with parthenogenesis, one of the characteristic 

 features of the abnormal cases being a weakness or absence of synaptic 

 interaction of chromosomes in the meiotic prophase. ^^ To what extent 

 this abnormal synaptic behavior is due to hybridity in the animals 

 concerned, as it often appears to be in plants, remains to be determined. 



^^ J. Harrison (1920) and Harrison and Peacock (1926) on Tephrosia; Schrader 

 (1925) on Daphnia; Vandel (1928) on Trichoniscus. 



^^ See de Baehr (1920a) on Aphis and Fries (1909) on Artemia. 



