SPINDLE DEVELOPMENT AND BEHAVIOR IN THE GIANT AMOEBA

Abstract

The chromosomal fibers of the giant amoeba appear during prophase, approximately at right angles to a loosely organized chromosome plate. At late prophase they are more closely grouped and slightly arched. This arching is observed during metaphase and early anaphase, the fibers becoming straight at mid-anaphase. They do not change materially in length at any stage of nuclear division. Chromosomal fibers become increasingly closely packed as the diameter of the chromosome plates decreases.Interzonal spindles are straight at early anaphase, barrel out at mid-anaphase, and appear relaxed at late anaphase and telophase. They reach a length of 65 microns, but spindles more than 45 microns long appear rope-like and individual fibers cannot be distinguished. When groups of daughter chromosomes with their chromosomal fibers encounter an obstacle to their movement, chromosomal fibers are crumpled, and interzonal fibers buckle. This behavior, and the barreling of mid-anaphase spindles, must be interpreted as evidence that anaphase seperation in this organism consists of a pushing apart of the groups of daughter chromosomes by the elongation of the interzonal spindle.Interzonal spindles 18 or more microns long are consistently twisted to the right. Uncoiling of a spiral of constant direction within the interzonal fiber, accompanied by a lateral attraction between fibers, is offered as a possible explanation of the constancy of the direction of the twist.