In human embryology, what starts ovum cell division?
Actually an ovum doesn't ever start cell division because once it is fertilized by a sperm the combination of the two cells is call a zygote.  In humans, the zygote proceeds with the many steps following fertilization leading to the first division; this takes on the order of 24 to 30 hours.  At that point the set of cells may be termed an embryo or various other technical terms.  The succeeding cell divisions occur by the normal cell division mechanism, which is called mitosis.  In the context of the development of the human embryo, an early stage cell division is often called cleavage.

More details are available from the links in the above.  For greater technical depth on the initial stages of the division of the zygote, you might want to see this article.