Random thought...thanks to Disney

I saw a commercial for the new Disney movie (about the frog and princess) and one of the lines a character says in the preview is something along the lines of, "I want to be human! I want to have thumbs and a belly button!"

This made me think (because this is how I am), 'Hmm...in order to have a belly button, one has to have an umbilical cord.'

This got me investigating. Do all mammals come from placentas? Do animals like whales have belly buttons?! It's not something I have ever thought about (or remember learning in any class), because it seems like such a human thing (to have a belly button). :)

Anyway, all mammals are placental, except marsupials and monotremes.

Ok, so now I got to learn two additional things because of this!

1. There are mammals that LAY EGGS?! They are monotremes, e.g. platypus.

2. Marsupials have a "less-developed" placenta that limits their gestational period. True placentas allow for longer gestational periods and ultimately higher evolutionary success!

*All info was from Encyclopedia Britannica