sentences:  An internal yolk sac develops when the embryo is long, which begins to take in yolk as the external yolk sac shrinks; by the time the embryo is long the external yolk sac has been completely resorbed.Off Portugal, the young seem to be born in May and December following a gestation period of over a year.As they near giving birth, the females undergo ovarian atresia (regression of the follicles), suggesting that they enter a resting period afterwards.