 When the embryo is long, the yolk supply begins to run low and folds appear on the yolk sac and uterine wall, which later interlock to form the placenta.At this stage, the embryo has most of the features of an adult, albeit rudimentary and colorless.The blades of the cephalofoil are folded back along the body, and long external gill filaments protrude from the gill slits.