212 HEREDITARY TRANSMISSION [CH. 



numbers of gametes which have it and are without it ; or it 

 may be omitted from the daughter nuclei altogether, or in 

 some cases it may divide in the first and pass to one pole 

 without division in the second. In consequence of this 

 variable behaviour less than half the germ-cells possess the 

 odd chromosome, the number varying in different plants, 

 and correspondingly it is found that lata parents crossed 

 with the type form give a variable percentage of lata off- 

 spring, ranging from 4 to 45 per cent. GATES, to whom our 

 knowledge of the facts is chiefly due, has occasionally seen 

 abnormalities in the maturation of the pollen cells in the 

 type form of Oenothera by which both members of a pair of 

 chromosomes go to one pole in the heterotype division, and 

 he supposes that the fifteen-chromosome (lata) races arise 

 in this way. It is not known whether the extra chromosome 

 is always homologous in different lata races, or whether the 

 duplication of any chromosome produces a lata form ; since 

 there is another variety (semilata) that also has fifteen 

 chromosomes, it is possible that the duplication of one 

 chromosome produces lata and of a different one semilata. 



Another variety of Oenothera that is correlated with a 

 difference in the chromosome number is the form gigas, 

 which has twice the normal number, that is to say, it is 

 "tetraploid" instead of diploid, and has 28 chromosomes 

 in its somatic cells. In this form also there are irregularities 

 in the maturation divisions, leading to frequent sterility. 

 Another somewhat similar case of a tetraploid variety in 

 Primula will be described after the possible relation between 

 chromosomes and Mendelian characters has been con- 

 sidered. 



In the examples given above there is more or less direct 

 evidence, even though it is not always as satisfactory as 

 might be wished, of a connection between one or more 



