454 



BREAKDOWN OF GENETIC SYSTEMS 



pseudogamous Potentilla species. In spermatocytes produced in the 

 ovary of a parthenogenetic Rhahditis (Belar, 1923) meiosis is also 

 not completely irregular ; some pairing of chromosomes is found at 



MI R-N MU TjX 



^Embryo ^^ Em btyo 



Sac 



Fig. 131. — Development of the embryo-sac in Ixeris dentata by 

 parthenogenesis : prophase — metaphase (21 univalents) — 

 restitution-nucleus — second metaphase and telophase. Finally, 

 the embryo in the two-cell stage (after Okabe, 1932). 



the first metaphase, and the sperm formed actually functions in 

 stimulating the a^g to development. 



Amongst plants with obHgatory apomixis the pollen mother-cell 

 behaviour is often closely parallel with that in the embryo-sac 



