• Converts unicellular zygote to multicellular embryo 
• Produces many cells that can interact and be moved around 
• Maintains diploid complement of cells--all are genetically identical 
• Human cleavage is not synchronous; all of the cells do not cleave at precisely the same time, as a result embryos with odd numbers of cells can be seen at various times 
• Slow cleavage; takes approximately 12-24h between each cell division 
• No growth occurs during early cleavage, so the total embryo will remain ~100 microns in diameter 

Once the egg is fertilized, meiosis of the female nucleus continues resulting in the female pronucleus followed by fusion of the male and the female pronuclei. The female and male pronuclei are clearly visible in the photograph below.

This information was provided by Professor Danton H. O'Day - Univerisity of Toronto at Mississauga. Unless otherwise stated the information and graphics that are presented within this embryology section are the sole property of Danton H. O'Day, copyright 1998(c), 1999(c), 2000(c), 2001(c), 2002(c).
 

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