CoachIn animals, making young usually takes two parents: a female and a male. The female produces egg cells; the male produces sperm cells. A young animal begins when one sperm cell joins one egg cell — that joining is fertilization. Reproduction with two parents is called sexual reproduction.
示例
Example 23.2 (Why the frogs sing)
The May chorus is the males calling the females to the pond. Each pair then releases its cells into the water — the female’s eggs, the male’s sperm — where fertilization happens. The black dots in the jelly a few days later are the young already growing: the song was step one of Chapter 8’s frog cycle.