Egg

Mammal milk evolved from skin secretions that originally kept eggs moist. The platypus, the only surviving egg-laying mammal, still coats its soft eggs with secretions to prevent drying, and its hatchlings feed on these secretions for nutrition.

The kiwi bird lays an egg weighing about a quarter of its body weight—proportionally the largest egg in the world. Thanks to this enormous egg, kiwi chicks hatch nearly fully developed and can survive on yolk nutrients for about two and a half weeks after birth.