A platelike structure by which the microtubules of the spindle attach to the centromere of a chromosome during nuclear division. In higher organisms it consists of protein and RNA arranged in three layers closely apposed to the chromatin. It acts as a motor, pulling the centromere along the attached microtubules towards the spindle pole. This process is thought to involve motor proteins, such as dynein, and disassembly of the microtubule subunits.