Submicroscopic structure of meiotic chromosomes during prophase

The existence of well differentiated structures in prophasic nuclei of rat primary spermatocytes has been demonstrated. These structures consist of ribbon-like groups of three threads; two external thick and dense (500 Å wide); one medial thin and light (120 Å wide). The external ones run equidistant to the medial filament (with a distance of 200 Å between them). The cross sections of the groups show that each filament is an independent unit. As the position of the external threads was seen changing from side to side of the medial one, it is suggested that they may have a wide helicoidal course. A number of them were seen contacting or attached to the inner sheet of the nuclear membrane. They were found existing only in the preparatory stages of meiosis.