Protoplasmic Structure and Mitosis

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Abstract
The mitotic figure of the sea-urchin egg is most strongly birefringent at metaphase. During anaphase this birefringence decreases considerably, but the spindle and asters both grow in size. These changes have been investigated quantitatively by constructing curves of retardation and coefficient of birefringence across the mitotic figure, using techniques described in an earlier paper. The decrease of birefringence in the spindle starts at the equator, and then moves, in the course of a few minutes, to either pole. Only when the decrease has reached the spindle poles does it begin in the asters, where it moves outwards from the centres. These changes resemble the movements of the chromosomes, which also start at the equator in metaphase, and move in separate groups to the poles during anaphase. By examining single eggs in anaphase up to the moment of fixation, and then staining them to show the chromosomes, it is established that the regions of, decreasing birefringence actually correspond to the position of the chromosomes. Since the chromosomes are too small to be the direct cause of the decrease in birefringence, it is concluded that they are producing the decrease indirectly by initiating a structural change in the spindle and asters. The possible mechanisms for this change are discussed. It is concluded that the chromosomes must be releasing an active substance, for which the term ‘structural agent’ is suggested. The growth of the mitotic figure takes the form, in the spindle, of an increase in length, and in the asters, of an increase in size. It is accompanied by an increase in coefficient of birefringence, though this is to some extent masked by the decrease in birefringence referred to earlier. The increase in coefficient of birefringence affects the whole mitotic figure from the very beginning of anaphase, and is not therefore relatable to the position of the chromosomes. For this reason it might be due to a number of different mechanisms, but as it starts at the same moment as the decrease in birefringence, it is tentatively assumed to be due to the release of a second ‘structural agent’. The increase in coefficient of birefringence is probably due to the orientation of new material. The decrease is more likely to be due to changes in molecular and micellar arrangement; it would be consistent with a contractile mechanism in the spindle. The implications of these findings are discussed in a concluding section.

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