The mitotic cell cycle: explain question

A scientist observes two cells from an animal tissue under a light microscope. In cell X, the chromosomes are visible as two sister chromatids joined at the centromere, and the nuclear envelope appears fragmented. In cell Y, the chromosomes are aligned at the equator of the cell, and fine threads (spindle fibres) extend from the poles towards the centromeres. Explain the differences in chromosome behaviour and the associated structures of the nuclear envelope and spindle between the two stages represented by cell X and cell Y.