Embryo Stage 4-5 weeks Embryonic Stage begins The Yolk sac is present and the neural plate and folds are present. The optic vesicles are present. Failure to visualize yolk sac after 42 days associated with empty sac. Embryo CRL 4 mm.
Limb buds are present.
The primordia of liver, pancreas, lungs, thyroid gland, and heart are present. The two heart tubes fuse in the mid-line and contractions commence. There is unidirectional blood flow. The yolk sac is seen within the gestational sac. Double-blep sign, first evidence of amnion as double line adjacent to the yolk sign.