It takes about 175, 000 sperm cells to weigh as much as a single egg cell.
They mistakenly thought that the sperm cell contained a miniature preformed human Being that grew when it was deposited in the female genital tract.
If the egg is fertilized by a sperm cell, it stays in the uterus and grows into a baby, using that extra blood and tissue to keep it healthy and protected as it's developing.
The breakthrough came when some cells continued to grow, elongating and growing a tail which caused them to move, and forming recognisable sperm cells.
These cells can be stored and multiplied in culture, and are theoretically capable of making any tissue in the body - including sperm cells that could be used in assisted captive breeding programmes.
