Animals that lay eggs don't have belly buttons

As weird as it sounds, the belly button once served a purpose for mammals. It is where the umbilical cord attaches to the baby inside the mother’s womb. The umbilical cord provides nutrients from a mother to a baby mammal, allowing the baby to grow from a fetus to a full-grown baby ready to be born. In egg-laying animals, the baby never grows inside the mother, and therefore there is no need for the umbilical cord to form and nutrients to be passed along from mother to offspring. This means the bellybutton never forms.