By the end of this week, your baby measures about one-third of an inch from crown to rump. (Doctors use this "sitting height" to track early growth because baby's legs are hard to measure). Curled up, she's about as big as a pea. But she's a pea with personality: her facial features are starting to emerge -- small hollows that will become eyes, inner ears, and the beginnings of mouth and tongue.

Around the embryo's tiny body, a first layer of skin forms, delicate and translucent. The ends of the arm buds flatten into little paddles called "hand plates." By the end of next week, fingers will have formed. In this early stage, they are webbed, like duck's feet.

Amazing changes are taking place in the embryo's brain right now. Its cells are dividing into three sections. First is the forebrain, home base to the human personality. This is where problems will be solved and the avalanche of information gathered by the senses -- sounds, smells, tastes -- will be processed. Next comes the midbrain, which will serve as a relay station, coordinating messages and sending them on to their final destinations in the body. Last is the hindbrain, responsible for regulating the heart, breathing, and muscle movements. All three parts, working together, form the miraculous command center that is the human brain.

Hard to believe: though he's no bigger than a lentil, your baby already has a beating heart! By the middle of this week, his cardiac muscles have formed and begun fluttery little flexing movements. This mini -version of the human heart is your baby's first working organ.

For now, this little being still looks more like a curled-up tadpole than a human baby. A huge head makes up almost half the embryo's length (the better to accommodate a rapidly growing brain). In the midsection, near the bulge that holds the heart, tiny arm and leg buds have appeared. Just below them is the attachment point of the umbilical cord, slender as a thread. The cord connects the fetus to the growing placenta, the life-giving organ that has formed to supply the growing baby with the blood supply and nutrients needed for survival.

The body ends in a long tail. But don't worry--this reminder of man's evolutionary roots disappears within a few weeks, absorbed into the spinal cord.

Inside the body, the central nervous system, the circulatory system, and organs like the liver and pancreas are developing. Along the embryo's "back" is a row of small bony blocks called somites. These will become the spine and ribs, among other things. [LINK]