• 0.1 - 0.15 mm in diameter
• Characteristic feature is unicellularity.
• Fertilization is the procession of events that begins when a spermatozoon makes contact with an oocyte or its investments and ends with the intermingling of maternal and paternal chromosomes at metaphase of the first mitotic division of the zygote.
• Fertilization requires slightly longer than 24 hours.
• Takes place in the ampulla of the uterine tube.
• Fertilization includes:
  1. Contact of the spermatozoa with the zone pellucida of an oocyte, penetration of one or more spermatozoa through the zona pellucida and the ooplasm, swelling of the spermatozoa head and extrusion of the second polar body.
  2. The formation of male and female pronuclei.
  3. The beginning of the first mitotic division, or cleavage, of the zygote.

Question: How long a time period does fertilization require?

Developmental Stages in Human Embryos by Ronan O'Rahilly and Fabiola Müller.  Published by Carnegie Institution of Washington, Publication 637. 1987. Stage 1 (PDF)

Stages of Development

Stage 2