Short answers, pulled from the story.
William Harvey lived from 1578 to 1657 and proposed that every living animal comes from an egg. This doctrine rejected the idea of spontaneous generation and preformationism.
Karl Ernst von Baer discovered the mammalian ovum in 1827. This discovery proved Harvey correct about mammals but required a microscope to see the tiny cell.
Oskar Hertwig observed sperm fusing with ova of a starfish in 1876. The fusion created a diploid zygote that grew into a new organism.
The human ovum measures approximately one millimeter in diameter. It is one of the largest cells in the human body yet visible to the naked eye without magnification.
Oviparous animals include birds, most fish, amphibians, and reptiles. Their ova develop protective layers passing through the oviduct to the outside body.