Behavior of the Spindle Fibers in Centrifuged Cells

Abstract

As I described in a previous paper, 1 by centrifuging the cells in mitosis, in the onion root tip, for one hour with a force equivalent to 30,000 times the force of gravity, I obtained a displacement of the chromosomes in the centrifugal direction, a concentration of the cytoplasm in 2 layers at the lower portion of the cell, and a separation of the cell sap to the upper region. What happens to the mitotic spindle when the cells are treated under the same conditions is the subject of this communication. The method used is, in all its details, the same as that already described. 2

In the spindles and spindle fibers so treated I observed the following facts: 1. Only the upper cone of the spindle is present (Fig. 1, A-E), the lower one not appearing. 2. The height of the cone often exceeds one-half, and can be as much as three-fourths, of the height of the cell (Fig. 1, B). However, all the intermediate heights between this maximum and the short cones of the prophase stage are present. 3. I never observed fibers broken or badly distorted or folded on themselves, and rarely were they entangled. They are often slightly bent, but in a smooth curve. 4. The fibers stay together as the hair of a moist tapering brush; they are never scattered in different directions. 5. The half spindle bores its way centripetally through the heavier cytoplasm. It sometimes bends under the weight of the latter and seems to turn around cytoplasmic masses in its centripetal path (Fig. 1, D).

I venture the following tentative interpretation on these observations: The fact that only the upper part of the spindle appears in the picture should probably not be attributed to a crushing and destruction of the lower part, since there is usually no trace of it left; it is due rather to the centripetal displacement of the fibers of the lower cone when the heavy chromosomes to which they are attached move centrifugally.