How Belly Buttons Form
A belly button, scientifically known as the umbilicus, is a scar. This scar forms where the umbilical cord once connected a developing fetus to its mother. In placental mammals, including humans, this cord delivers oxygen and nutrients from the mother to the offspring and removes waste products.
After birth, the umbilical cord is cut, leaving a small stump. This stump naturally dries and falls off. The tissue that remains then heals, forming the characteristic depression or protrusion known as a belly button. This process is universal among placental mammals.
Egg Development and the Absence of a Navel
Animals that lay eggs, categorized as oviparous, undergo a fundamentally different developmental process. These animals, which include birds, reptiles, amphibians, and the vast majority of fish, do not form a placenta or an umbilical cord. Instead, their entire embryonic development occurs within an egg.
Within the egg, the developing embryo is nourished by a specialized structure called the yolk sac. This sac contains all the necessary nutrients for the embryo’s growth. The embryo absorbs these nutrients directly from the yolk, meaning there is no need for an external attachment that would later result in a scar like a belly button.
Temporary Marks and Unique Cases
While oviparous animals do not possess a true belly button in the mammalian sense, some may exhibit a temporary mark or scar where the yolk sac was absorbed or detached. This small, often subtle mark differs significantly from the mammalian structure. For instance, newly hatched birds often have a minuscule residual scar where the yolk sac was incorporated into their body. This mark typically fades quickly and becomes invisible as the chick grows.
Similarly, many reptiles, such as snakes and lizards, may have a faint scar from their yolk sac attachment. In most cases, this mark closes and becomes imperceptible as the animal matures. However, some reptiles, like alligators and crocodiles, can retain a more visible umbilical scar that persists into adulthood. Even certain fish species might display a temporary mark from their yolk sac, which also disappears as they grow larger. The monotremes, an unusual group of egg-laying mammals like the platypus and echidna, also lack a belly button because their embryos develop inside eggs, relying on a yolk sac for nourishment rather than a placenta and umbilical cord.