There are two co-ordinate processes in the cleavage (or cell division) that is displayed in Figure.
The mitotic division of nucleus depends on the creation of mitotic spindle that is constituted by microtubules of which the tubulin protein is the structural unit Treatment of the egg along with the drug colchicines disrupts the microtubules and arrests Karyokinesis at metaphase.
Figure: Role of microtubules and microfilaments in cell divisions
Division of the cell depends on the contractile microfilaments of which protein actin is the structural unit. A ring of microfilament emerges in the cortex around the cell in which the cleavage furrow is formed. Contraction of the microfilament ring in the purse-string manner deepens the furrow finally cutting the cell into two. Treatment of the egg along with cytochalasin B inhibits the organisation of contractile ring of microfilaments that is why cleavage furrow is not made and cytokinesis does not take place.