In sexual reproduction, two parents reproduce offspring. The offspring are similar but not clones. The female produces a sex cell called a(n) egg. The male produces a sex cell called a(n) sperm. When the two join, a developing structure known as a(n) embryo is created. This process is called fertilization. The embryo will grow to become an adult, with traits from both parents. Most mammals give birth to fully developed young. However, the platypus is one mammal that lays eggs.