Ciné-micrographic studies on mitosis in endosperm. IV. The mitotic contraction stage

Ciné-micrographic studies on mitosis in endosperm: IV. The mitotic contraction stage

• 1.1. The nuclear membrane breaks down when the clear zone reaches its maximal size. Just after the membrane disintegrates and immediately before the contraction stage, the nuclear sap begins to mix with the substance of the clear space. The rupture of the nuclear membrane initiates the contraction stage.
• 2.2. After the loss of the mechanical barrier provided by the nuclear membrane, chromosomal limbs at the periphery of the group straighten slightly, so that the chromosomes come to occupy a slightly greater apparent area. Then follows a reduction in the space occupied by the chromosomes: the contraction stage. This apparent compression of the group is followed by simultaneous straightening of chromosome limbs and stretching of kinetochores. After this stretching the kinetochores begin to lead the movements of the chromosomes; it is suggested that the kinetochores are stretched when they first become attached to the spindle. During contraction stage the spindle gelifies, after which an orientated structure is manifested.
• 3.3. The extent of the irregular movements of small particles decreases after the contraction stage; when the particles begin to oscillate parallel to the long axis of the spindle.