Prime Types in Datura.

in regard to aggregate constitution. The several types of recurrent mutants known to involve change of configuration (such as half-mutants and mass-mutants) correspond with these in frequency of origin and in gametic constitution. The fact that the crossing over which yields them is rare is not surprising since crossing over is regularly found to be reduced in structural heterozygotes in Drosophila.7 I conclude therefore that (a) the occurrence of relatively translocated segments, (b) their permanent separation in the history of opposite complexes, and (c) their association with reduplications and deficiencies within each gametic complex, all of which were inferred from the genetical and cytological properties of complex heterozygotes may now equally be inferred from the conditions of interchange under which these heterozygotes must be supposed to have arisen. * The higher frequency of non-disjunction in maize than in Oenothera is also discussed by Emerson. Comparison with Pisum and Campanula shows that it can be attributed to the greater rigidity of configurations with interstitial chiasmata.8 I Emerson, Sterling, Proc. Nat. Acad. Sci., 18, 630-632 (1932). 2 Darlington, C. D., J. Genet., 24, 405-474 (1931). 3 Darlington, C. D., Recent Advances in Cytology, 142-143 (1932). 4 McClintock, B., Univ. Miss. Ag. Exp. Sta. Res. Bull., 163, 1-30 (1931). 5 Sansome, E. R., Cytologia, 3, 200-219 (1932). 6 Catcheside, D. G., Ibid., 3 (in the press), 1932. 7 Dobzhansky, Th., Am. Nat., 65, 214-232 (1931).