Effects of demecolcine upon the mitotic apparatus of sea-urchin eggs

Abstract

Morphological alterations of the mitotic apparatus and blocking of the cleavage caused by application of demecolcine to eggs of Mespilia globulus at various stages of the 1st nuclear division are described. By treating fertilized eggs with the poison at stages before anaphase, the development of the mitotic apparatus is prohibited or the apparatus already formed disintegrates. No sign of furrowing appears and the cleavage is completly blocked in these cases. Contrary to these, furrowing commences and advances quite normally in eggs which are exposed to demecolcine at any stages of nuclear division later than the early anaphase, notwithstanding that astral rays disappear within one or two minutes after the application of the poison. These facts would suggest that the critical period lies between the late metaphase and the early anaphase, and when the process of nuclear division proceeds beyond this stage the cleavage begins and accomplishes itself quite normally in the absence of the mitotic apparatus. The mechanism of cleavage is discussed in connection with the present finding.