The mitotic cell cycle: explain question

A student treats actively dividing onion root tip cells with colchicine, a chemical that inhibits the polymerisation of tubulin into microtubules. When observed under a microscope, the treated cells show condensed chromosomes scattered throughout the cytoplasm, no visible spindle fibres, and the absence of a nuclear envelope. With reference to the stages of mitosis, explain why the cells have not progressed to metaphase and describe the normal behaviour of the spindle and nuclear envelope during the stages that occur prior to the observed block.