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.

By the time the embryo is six months old (pictured), it has a sense of smell 10,000 times sharper than a human's. This and other sensory adaptations--including electro-sensors that detect the faint voltage of other animals--will one day allow the shark to detect even a fin flick hundreds of feet away, according to the new National Geographic documentary In the Womb: Extreme Animals.

Related Video: Lemon Shark Gestation and Birth

ON TV In the Womb: Extreme Animals airs Saturday, May 16, 2009, at 9 p.m. ET/PT on the National Geographic Channel. Details >>