Cavities form to define the amnion and yolk sac. This is the first sign of an organ, long before heart, GI tract, muscles, or other organ systems form. Folding So far, embryo has been living on stored energy in egg. Now, it begins to feed on mother's tissues through placenta. At end of 8 weeks, embryo is about 1 inch in length, weighs 1 gram, all organs are already visible.
Heart is the size of a poppy seed, and has begun to beat.
Embyro is about 1 inch long . Fingers are visible but still connected by webbing. Veins are visible. Heart has divided into left and right chambers. Fetus is about 3 inches long and fully formed.