The Number of Chromosomes 387 



In recent years we have discovered further that the 

 number is not so constant, after all. Thus, we have seen 

 that the number may be different in the two sexes. Now 

 it is found that other differences are also present. In some 

 groups of plants and of animals there appear individuals, 

 and whole strains, in which the number of chromosomes is 

 triple or quadruple the number found in the germ cells, in- 

 stead of simply double (page 253). Or the number of 

 chromosomes may be an even larger multiple of the normal. 

 In other cases there appear individuals in which the nucleus 

 of the cells shows one or more supernumerary chromosomes, 

 or a shortage of one or more chromosmes. 



Experimental breeding, combined with the microscopi- 

 cal study of the chromosomes in the plants and animals used, 

 shows that there is a close relation between mutations and 

 these two types of irregularities in the chromosome num- 

 bers — that is, the addition or subtraction of one or a few 

 chromosomes, and the multiplication of the entire sets of 

 chromosomes. 



The evening primroses supply examples of both kinds 

 of irregularities. In one series the cell nucleus contains one 

 or two supernumerary chromosomes. In each case the in- 

 dividuals present a correlated set of body characters that 

 affect all the organs of the plant. The normal number of 

 chromosomes in the ancestral type, lamarckiana, is seven in 

 each germ cell and double this number, or fourteen, in each 

 body cell. In certain new species, however, each body cell 

 contains fifteen chromosomes. These eight species are dis- 

 tinct from each other as well as from the parental form: 

 sci7ttillaiis, albida, oblonga, subovata, cana, pallescens, 

 lactua, liquida. Since there are only seven chromosomes in 

 the germ cell, it is conceivable that seven mutations 

 might arise by the duplication of one or another of the 

 chromosomes. Since there are more than seven of such 

 primary mutations, it is possible that two supernumerary 

 chromosomes sometimes take part in the formation of a 

 mutant. 



