In the prophase stage, the chromosomes become visible, the nucleolus disappears, the mitotic spindle forms, and the nuclear envelope disappears.

Concept 5: Prophase
Concept 5: Prophase, Pearson Education, Animated images and text explaining prophase and prometaphase.
http://www.phschool.com/science/biology_place/bioc...
During mitosis what happens in prophase?
During mitosis what happens in prophase?, WikiAnswers, Answers.com, Visit the web page for the answer.
http://wiki.answers.com/Q/During_mitosis_what_happ...
Mitosis: Prophase and Prometaphase
Mitosis: Prophase and Prometaphase, SpartNotes.com, Illustrations and description of the first and second stages of mitosis.
http://www.sparknotes.com/biology/cellreproduction...
Stages of Mitosis - Prophase
Stages of Mitosis - Prophase, By Regina Bailey, About.com, Text, image, and illustrations explaining prophase.
https://www.thoughtco.com/stages-of-mitosis-373534...