 The spadenose shark has the most advanced form of placental viviparity known in fish, as measured by the complexity of the placental connection and the difference in weight between the egg and the newborn young.Newly ovulated eggs measure only in diameter, while the developing embryos become dependent on their mother for sustenance at a length of only .The placental stalk, formed from the yolk sac, has an unusual columnar structure and is covered by numerous long appendiculae that support a massive capillary network, providing a large surface area for gas exchange.