1907] GATES— HYBRIDIZATION OF OENOTHERA MUTANTS 13 



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of the individual and has persisted in all subsequent cell generations. 

 Of course there is the possibiKty that a higher number of chromo- 



somes mi 



mentation of certain chromosomes; but no constant size-differences 

 have been found among the chromosomes of any of the forms or 

 between the chromosomes of the different races examined. 



There are a number of ways in which it would be possible for 



F 



fusion nuclei having twenty-one chromosomes to be formed during 

 the period of fertilization, since the gametophyte number of chromo- 

 somes is seven. 



(i) It is possible that O. Lamarckiana might produce tu^o kinds of 

 pollen, having respectively seven and fourteen chromosomes. My 

 examination of the conditions of pollen development in O. Lamarck- 

 iana has thus far revealed no evidence whatever in favor of such an 

 hypothesis. Apparently all the pollen mother cells undergo the reduc- 

 tion divisions^ in which the number of chromosomes is reduced from 

 fourteen to seven in the usual way. 



(2) It is possible that O. lata might produce two kinds of eggs, 

 having respectively seven and fourteen chromosomes. If both these 

 kinds of eggs were fertihzed with O. Lamarckiana pollen and pro- 

 duced embryos, we should have plants resulting with fourteen and 

 twenty-one chromosomes. The difficulty here, however, is that on 

 such an hypothesis the union of seven lata chromosomes with seven 

 Lamarckiana chromosomes would produce a lata plant; while the 

 union of fourteen lata chromosomes with only seven Lamarckiana 

 chromosomes would produce a Lamarckiana plant, a situation which 

 >s highly improbable, to say the least. 



(3) Another possibility is that all the eggs of O. lata have the 

 unreduced number of chromosomes, and that part of them develop 

 without fertilization (parthenogenetically), producing O. lata plants 

 with fourteen chromosomes; while others are fertilized with O. La- 

 marckiana pollen, and produce Lamarckiana plants having twenty-one 

 chromosomes {fig. 3). This assumption is perhaps as reasonable as 

 any, but no case is known of an unreduced egg being fertilized. 



(4) Another possible source of twenty-one chromosomes is by the 

 Pinion of both male cells with the egg in fertilization. This, though 

 very unlikely, is at least a theoretical possibility. 



