IIS Gates, Tetraploid Mutants and Chromosome Mechanisms. 



as large. It is not clear from his figures whether there is an 

 accompanying difference in the size of cells and nuclei. Hybrids 

 between these species should be very favourable for tracing the 

 distribution of the chromosomes in meiosis. 



Another paper to which reference must be made, is that of 

 Artom (1911), who found similar conditions in the Crustacean 

 Artemia salina, the sexual form from Cagliari having 42 chromo- 

 somes, while the parthenogenetic form from Capodistria possessed 84. 

 Of all the papers hitherto cited, in which the question is considered, 

 Artom's is the only one which refers the origin of the doubling 

 to the union of two diploid germ cells rather than to the region 

 of the fertilized egg 11 ). 



Keeble (1912) has given an interesting account of the 

 origin and structure of a giant Primula sinensis wich originated 

 as a mutant in a culture of "White Queen Star". The cells were 

 found to be gigantic though the chromosome number (24) remained 

 unchanged. Gregory (1909) found the same relation to exist in 

 normal and giant Star Primulas. Keeble (1. c., p. 172) suggests 

 that the cell giantism "may be due to a reduction in the normal 

 rate of cell-division''. It seems to me more probable that the 

 original change or mutation was in the size of the cell, and that 

 the slower rate of growth was a secondary effect. This view is 

 based on the fact that Oenothera gigas, in which the chromosome 

 number is doubled, is also slower in its rate of growth. 



Several most interesting parallels to the case of Oenothera 

 giyas are furnished by the Primulas recently investigated by Miss 

 Digby (1912). The two species, P. floribunda and P. verticiUata, 

 each have 18 chromosomes as 2X number. P. floribunda X P- v&r- 

 ticillata gave the hybrid P. ketvensis which produced only thrum 

 flowers and was therefore sterile, having also 18 chromosomes like 

 the parents. It was multiplied by cuttings for about five years, when 

 a single pin flower appeared on one individual. This was pollinated 

 from a thrum flower and gave rise to the fertile race of P. keu'ensis, 

 having 36 chromosomes, from which a variety, 7^. keu-ensis farinosa, 

 having also the tetraploid number of chromosomes, was afterwards 

 obtained by selection. 



Of equal interest is the further fact that the reciprocal cross, 

 P. rerticiUata X P- floribunda i*beHina, also gave P. kewensis 

 farmoaa, having 36 chromosomes. Thus the doubling is not a 



11) I must also refer here to a recent observation of Geoffrey Smith (1912) 

 in which he found (in accordance with Guyer's results) that in hybrid pigeons 

 normal synapsis failed to take place in spermatogenesis, the chromosomes failing to 

 form bivalents and being irregularly distributed in the heterotypic mitosis. The 

 homotypic mitosis was almost wholly suppressed, the secondary spermatocytes forming 

 directly spermatids and spermatozoa, which are therefore of twice the normal size. 



