Emperor Penguin: Inside the Breathing Egg
Unlike developing humans, a baby penguin can't tap into the oxygen in its mother's bloodstream. Instead, its umbilical cord is linked to blood vessels in a membrane attached to the egg's inner wall. Oxygen enters the embryo's blood via microscopic holes in the shell--turning the egg into a kind of surrogate lung.
After 64 days of development, the baby penguin will slowly smash its way out, and its mother, with any luck, will be waiting with fish.
Lemon Shark: Sac to Stem
Shown about halfway through its 12-month gestation period in a computer-generated illustration, a lemon shark--like a human embryo--is literally connected to its mother via an umbilical cord attached to a placenta. But it wasn't always so. Until about three months in the womb, baby sharks feed off a yolk sac. Once the embryo has depleted the yolk, the collapsed sac settles against the womb wall and shoots blood vessels into the wall, tapping into the mother's circulatory system.