Mammals have a belly button which is the remainder of their attachment to umbilical chord but do birds reptiles fish etc. yolk sack?

You're on the right track, but it's a bit more nuanced than just a "yolk sack" for all those creatures. Here's a breakdown:

Mammals:

* Belly button: This is the scar left behind after the umbilical cord, which connected the fetus to the placenta, is severed after birth.

* Function: The umbilical cord carried nutrients and oxygen from the mother to the fetus, and waste products from the fetus back to the mother.

Birds, Reptiles, Fish, and Other Non-Mammalian Animals:

* Yolk sac: This is the sac within the egg that contains the yolk, which provides nutrients to the developing embryo.

* Function: The yolk sac is absorbed by the developing embryo as it grows, providing nutrition and energy.

* No belly button: Unlike mammals, these creatures don't have a placenta or umbilical cord. Therefore, they don't have a belly button.

* Scar: There might be a small scar where the yolk sac was attached, but it's usually very inconspicuous and doesn't resemble a belly button.

Key Differences:

* Placenta: Mammals have a placenta, a specialized organ that connects the mother and fetus and allows for nutrient and waste exchange. Non-mammalian animals don't have a placenta.

* Umbilical Cord: Mammals have an umbilical cord, which carries nutrients and waste products between the mother and fetus. Non-mammalian animals don't have an umbilical cord.

Therefore, while non-mammalian animals have a yolk sac, they don't have a belly button because they don't have the same type of placental development as mammals.