The longest gestation period for any animal is that of the spiny dogfish, Squalus acanthias , at 22-24 months. Examples of species that reproduce by ovoviviparity include the Lamnids like sand tigers and white sharks. Sand tigers have a unique variation of ovoviviparity called intrauterine cannibalism. Two young are born, each one meter long from a female that is only 3 meters in length. Great whites are also thought to exhibit intrauterine cannibalism.
Viviparous sharks form a placental from the embryo to the oviduct wall.
An embryo inside of an oviduct. The yolk in the yolk sac is first consumed by embryo, then uterine milk produced by female is absorbed by an embryo through the means of a placenta that is formed from the yolk sac to wall of uterus. Sharks are born live, and ready to fend for themselves outside the female. Oviparous sharks lay egg cases. The embryo feeds on the yolk sac inside.