This phase is of shortest duration. It begins with a sudden separation of the two chromatids of each due to splitting of its centromere and then a slow movement of separted chromatids towards opposite poles of mitotic spindle . Factors responsible for splitting of centromeres and movement of attachment fibres, or the fibres gradually shorten, pulling the chromatids towards the poles. Shortening of these fibres may be due to their contraction (due to the presence of contractile actin proteinin these ) or their gradual depolymeriaztion at polar ends Simultaneously, however the continuous spindle fibres somewhat elongate, pushing the spindle poles more apart. Towards the end of anaphase when polar migration of chromatids is complete , these is a full chromosomal complement at each pole identical to the original karyotype of parent cell. Soon, a nuclear envelope forms around each polar group, presumably from the endoplasmic reticulum.