1. The genetical and environmental control of major coiling has been studied in Tradescantia from a quantitative point of view. The number of major coils per chromosome was shown to be under complex genetical control. 2. The use of heat treatments revealed that the coiling cycle during meiotic prophase could be broken down into two stages: (1) the initiation of the coils, the spiralization phase; and (2) the loss of coils, the despiralization phase. It was thus possible to show that the major coils of metaphase are derived from numerous coils of small gyre size which are progressively reduced in number by uncoiling at the same time that the gyres are increasing in diameter. Using temperature from 3.5⚬ to 42⚬ C., it was found that at high and low temperatures the despiralization process was accentuated, leading to greater contraction of chromosome length. The greatest number of major coils was found at 27⚬ C. The effect of temperature on the number of major coils was considered both from the standpoint of ...