Spindle of Cell Division

achromatinic, or mitotic, spindle, a formation that arises in the plant or animal cell during its mitotic division (mitosis) and participates in the separation of the chromosomes. The spindle is a part of the mitotic apparatus; it consists of two types of cytoplasmic threads: central ones, which connect both poles of the cell, and chromosomal ones, which extend from the poles to the chromosomes. (The part of the chromosome to which the thread of the spindle is attached is called the centromere, or kinetochore.) The spindle threads are tubular formations approximately 200 Å in diameter. They have double refraction. Separation of chromosomes is due, on the one hand, to shortening of the chromosomal threads, and on the other, to lengthening of the central threads of the spindle. The mechanism of this phenomenon has not yet been clarified.