Analysis of cleavage stimulus by means of micromanipulation of sea urchin eggs

Abstract

The roles of the spindle, the aster, the nucleus and the centriole in the formation of cleavage furrow were investigated by examining the effects of removal of these structures on the process of the subsequent cleavage in blastomeres of sea urchin eggs at the two-cell stage. The results are summarized in fig. 2. In the cell from which one of two asters had been removed, a monaster developed in place of the diaster in the majority of cases, while two asters were occasionally formed. In the cell from which two asters had been removed, no aster was formed in some cases and a monaster was formed in other cases. Monaster formation was observed in a cell from which the nucleus and one of two asters had been removed. In the cell from which the nucleus had been removed, two asters were generally formed while a monaster was formed in some cases. In the cell from which the whole mitotic apparatus had been removed, neither aster nor nucleus was formed. In these experiments, cleavage furrow always appeared between two asters when they were formed in the cell. When the operations were carried out in the early stages of mitosis, cleavage furrow was generally not formed, either in cells with monasters or in cells without asters. When the operations were made after a certain interval, a cleavage furrow was formed in cells with monaster as well as in cells without mitotic apparatus. The presence of nuclear materials or spindle is not indispensable for the formation of cleavage furrow and for complete cytoplasmic division. The above results are explained by assuming that the aster is always formed around the centriole if it is present at the time of mitosis and that cleavage furrow is formed by the contraction of the cortex induced by a substance or substances released at the tips of astral rays.