Links
[1] https://hootsgo.org/?q=node/6351
[2] http://en.wikipedia.org/wiki/Sexual_reproduction
[3] https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/reproduction-methods-237/asexual-reproduction-and-sexual-reproduction-885-12136/
[4] http://science.jrank.org/pages/6100/Sexual-Reproduction-Animal-reproduction.html
[5] http://www.saburchill.com/chapters/chap0031.html