Human embryo at 6 weeks – only a half inch long!

At six weeks a translucent form reveals the dark mass of heart and liver at its center.

Now a half inch long, the embryo floats in the cushioning waters of a sac known as the amnion. Still tethered like a balloon to the embryo, the yolk sac is a measure of the embryo's progress: The sac manufactured blood for the embryo until its own liver assumed the task during the fifth week; the bone marrow will ultimately take over this function. The yolk sac, now useless, will soon detach and begin to shrink and harden. (National Geographic)