Progress in building large rings (Sec., News Letter 20:16. 1946).

The different rings of six chromosomes produced as the first stop in the program were backcrossed to normals; the progeny were grown and examined for pollen sterility. In each case plants approximately 75 per cent sterile were identified. These should be carrying the crossover which combines the two parental translocations in one gamete. Similarly backcrosses of the F 1 Θ10 from 1‑5‑6‑7Θ 8 xΘ 4 were grown. It is hoped that the selected ears represent the desired crossovers, but the sterility classes were more difficult to distinguish by the �pocket microscope" method used in the field.