54 MITOSIS : THE VARIATION OF THE CHROMOSOMES 



therefore find out from a comparison of different species, races, 

 mutants and hybrids what differences in the chromosomes can be 

 attributed to changes in their permanent structure and what 

 differences are due to a different joint reaction of the genotype 

 controlhng the chromosomes as a whole. Both these kinds of 

 difference will, on the chromosome theory of heredity, be due to 



Fig, 12. — Mitotic metaphase in pollen grains from different flowers of 

 Tradescantia brevicaulis, showing large and small chromosome 

 types. A small chromosome is seen in the left-hand plate 

 that has probably been lying in a separate nucleus, a persistent 

 nucleolus in the right-hand one. x 1800. Cf. Table 47, p. 318 

 (after D., 1929 c). 



changes in the arrangements or properties of genes, but the first 

 will show in the part directly affected while the second, being due 

 to gene mutation, will not be visible in direct effect at all, but will 

 be seen by its altering the characteristic form or behaviour of the 

 complement as a whole. Since both changes are genetic, the second 

 kind will be distinguished as genotypic, for it takes effect through 

 the action of the genotype as a whole (D., 1932 a). 



