Chalazae:
Ever noticed those little springy white cords attached to egg yolks? Have a look next time you crack an egg. They’re the chalazae and they hold the yolk suspended mid-egg, so that it floats in an incubation bath with a constant temperature. They also ensure the developing chick is not damaged when its mother turns the egg. Egg-turning only evolved in modern birds. Opposite birds buried their eggs, so it is possible their eggs didn’t have those stringy bits. [“Flipping the birds,” sidebar, Jeff Hecht, NewScientist (4 March 2017, paywall)]
