CHROMOSOME STRUCTURE X. AN X-RAY EXPERIMENTt

T WAS recently found (NEBEL and RUTTLE, in press) that Tradescantia I reflexa Raf. shows four threads1 per chromosome during all somatic as well as during all premeiotic stages. Multiplication of threads occurs a t metaphase during somatic division (fig. I). In meiosis of T . reflexa the four threads multiply a t early interkinesis and do not do so again until the metaphase of the first pollen division (fig. 2 ) . The leptotene thread is four-partite. This corresponds to the four-partite structure of chromosomes in mitotic prophases. The chromosome at interkinesis contains eight threads per univalent while a t the quartet stage each chromosome again contains four threads. The orientation of these threads disregarding the coils is equidistant for only a short time during telophase. During the remainder of the cycle the chromosome consists essentially of two well-defined chromatids which can only with special precautions be resolved into longitudinal half-chromatids. This orientation is represented by figures I and 2. To demonstrate the quadripartite nature of the presynaptic or leptotene chromosome X-rays were used in order to determine whether such treatment might bring about lesions affecting only one of the constituent threads of a chromosome. If it is possible to influence the discrete threads individually, the cytological study of their subsequent behavior may well complete the demonstration of the quadripartite nature of the presynaptic chromosome. These lesions, while related to the peculiar spatial and functional order of the four threads in a chromosome, seemed to vary in type with variation in dosage, as will be explained below.