21. The newly hatched larvae are not very well developed and obtain nutrients from a yolk sac . 22. In " Berghia stephanieae " the development is lecithotrophic ( feed off a yolk sac ). 23. These live off the yolk sac for about twenty-four days before settling and starting filter feeding. 24. Early in development, the human embryo has three germ layers and abuts an embryonic yolk sac . 25. By the time the embryo is across, it has fully resorbed its yolk sac and external gills. 26. These two layers are sandwiched between two balloons : the primitive yolk sac and the amniotic cavity. 27. The external yolk sac is entirely absorbed when the embryo is long and close to being born. 28. Upon hatching, the benthic larvae are largely immobile and defenseless until the absorption of their yolk sac . 29. Active and term by a yolk sac . 30. As far as known, they are ovoviviparous, with the developing fetuses primarily sustained by large yolk sacs .
