The mitotic spindle of L strain fibroblasts, fixed with glutaraldehyde followed by osmium tetroxide, contains many 150- to 180-A tubules. They appear first in the cytoplasm. They extend from the centrospheres to the kinetochores, and from one centrosphere to the other. Only occasionally can points of continuity between the spindle tubules and the tubules of the centrioles be observed. The chromosomal insertion is by a means of a thin dense plate of the kinetochore. The total number of continuous spindle tubules is between 500 and 600. Occasionally, tubules appear paired. At anaphase, short lengths of individual spindle tubules possess a coating of a substance of high density midway between the poles. These parts of the spindle tubules aggregate to form irregular groups, comprising the stem-body, and, by becoming aligned into a plate, they form the mid-body.

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