11.   The sperm fertilizes the egg.

12.   A single sperm fertilizes an egg.

13.   And there Yanagimachi made his first major discovery -- a way to fertilize hamster eggs in the laboratory.

14.   Another idea would be to block sperm with a contraceptive gel containing proteins that would stick to the sperm protein, keeping the sperm from from fertilizing an egg.

15.   As she does so, the male releases a burst of sperm from a cloacal opening above the pouch, fertilizing the eggs internally.

16.   But given the difficulty of obtaining and fertilizing eggs in the laboratory, the alternative might be no babies at all.

17.   But opponents of the research believe that human life begins when an egg is fertilized, and they consider it wrong to destroy the embryos for research.

18.   But they can still fertilize an egg when they are directly injected into it.

19.   But then she would put it back and fertilize the egg and see if it developed normally.

20.   Called broadcast spawners, the shellfish releases millions of eggs and sperm into the ocean water, counting on water currents to mix and fertilize the eggs.