Whether they will become flatworms, lizards, or humans, embryos all develop in much the same way. The human embryo at about a month, when it is no bigger than a grain of rice, appears to be a tiny reptile, complete with tail.

Changes take place with amazing rapidity. About 21 days after conception the tiny, rudimentary heart begins to beat. At around 2 months, the embryo begins to acquire a clearly human aspect. The tail shrinks and toes and fingers become distinct. Inside the embryo, nerves feather themselves throughout the body and sex organs start to develop.

The months of pregnancy progress and the beauty and logic of the unfolding of life become clear. As though in a long, slow dance, eyes travel toward the front of the face while ears gradually climb up the head. Buds become limbs, toes and fingers emerge, facial features form—the fetus kicks, sucks his thumb, smiles.

After 9 months, this portion of your baby’s fantastic journey is complete. Everything lies in readiness for life outside of your protective womb—and the next stage of the journey.

Communication

The human nervous system has been called the world's most efficient network for transmitting messages. It begins to form at 18 days gestation and continues to develop until several weeks after birth, penetrating to the minutest regions of the body. Conveying information along a trunk and branch system of intertwining cells at jet speed, it connects the brain with nerve endings in the muscles, glands, and sense organs. At birth 10,000 taste buds in the mouth, 240,000 hearing units in the ears, and up to 50 billion light-sensitive points in the eye will deluge the system with data, as the baby begins to see, hear, and taste the world.

Developing from the third week on, the embryonic heart is built like a transformer, one of those toys that morph into something else, say, a rocket that unfolds and refolds into a superhero. Throughout pregnancy, it needs only to act like a single pump, maintaining blood flow throughout the body and into the placenta. Yet to meet the circulation requirements of life after birth, when oxygen molecules must be grabbed from the air through breathing, the embryonic heart must also form 4 chambers that can alternately pump and receive blood from the lungs. This physiological dilemma is solved by the presence of 2 shunts that allow each chamber of the heart to handle large amounts of blood while sparing the underdeveloped vessels in the lungs. These shunts automatically are sealed shut at birth, as the baby breathes - and cries - for the first time.

Development of the eye is first evident at about 22 days gestation, with the appearance of 2 symmetrical bulges - optic grooves - on the earliest brain-plate tissue. The human eye is an enormously complex structure. The cornea and the lens must be transparent and properly aligned to provide a pathway for light from outside to the retina. In turn, the retina - consisting of millions of light-sensitive cells - must be arranged in such a way as to receive visual signals and relay them to the proper parts of the brain. While all this is developing, during the second trimester, more than one million optic nerve fibers are growing outward from the brain to mesh with each eye. Tunneling through bony sockets, they somehow meet precisely. When they do, at about 6 months, nes become sensitive to various levels of light and darkness, but still cannot perceive objects.

HEARING AT 24 WEEKS: Among the senses, the ability to hear is what first tells us we aren't alone. Pinched, folded, furled, and grooved, the cornucopia-shaped ear is sculpted, gradually, in 3 sections from the embryo's thin skin and a prominence in the lower face. (Eventually it will be sensitive enough to discern whispers and durable enough to withstand cannon bursts.) At between 5 and 6 months, the fetus hears its first sounds -- the cacophony of its mother's body (digestive rumblings, rushing blood) and at the far margin of its world, her voice.

THE HEART'S MAP: The blood transports so many substances and does so many jobs it's hard to name them. Chiefly, though, it moves oxygen and food to all parts of the body while taking away waste chemicals. This process necessarily begins in the second week and by 20 days a system of watertight tubes branch throughout the embryo. The system grows “ahead" of the baby, proliferating in all directions, like highways on a busy map, stimulating new areas of development. (Understanding how blood vessels are laid down in relation to cellular reproduction has provided some of the most hopeful recent insights regarding cancer.) After birth this network also serves to provide the baby with central heat, distributing warmth evenly through the body, from busy parts like the heart to cooler areas like still muscles.

CLEANSING: Three successive sets of kidneys eliminate wastes from the blood, the last forming in the third month. These permanent organs must eventually strain 15 gallons of recirculating fluid daily, and it's estimated that they have 2 million microscopic filters to trap particles. Though the kidneys are fully formed in utero, they are among the first organs that must spring to life at birth, when wastes can no longer be passed on to the mother through the placenta. Heroically, many newborn babies will urinate as soon as their umbilical cords are cut.

SO EARLY: The rudiments of fingers appear at about 6 weeks, when the fetus is half an inch long. One week later, all the fingers are present, although not yet fully formed. As the hands spread wide during the next several weeks, fanning like the spikes on the Statue of Liberty's crown, or Lisa Simpson's hairdo, they began to open and close. Fingernails develop; the thumb grows out to oppose the fingers; tiny ridges for gripping (and later identification) form at the tips. By 13 weeks, the hand-which has 27 bones connected by a brocade of ligaments - is structurally complete. One month later, it possesses a firm grip, demonstrating muscular strength, and reflex action coordination. Thumb-sucking may begin around this time, sometimes so avidly doctors have reported delivering babies with calluses on one thumb.

ON TWO FEET: Bones are busy living tissues, and no part of the body has more of them than the foot – 26 in each, lashed together by tough, stringy bands of connective fibers. The baby's feet begin as paddlelike plates at the tips of the leg buds at around 6 weeks, when the embryo is still less than a half-inch long. (Nature builds appendages in the order in which it is going to need them; since the baby will have to grab and hold things before she finds a reason to stand and walk, feet begin to develop about a week later than hands.) By 13 weeks, the toes are articulated, the heel has developed, and the skeletal architecture is fully in place. A miracle of design, the foot is an elastic arch, flattening when you put it down and springing back to a curved shape when you lift it. This helps achieve the uniquely human walking style of swinging our legs in front and behind us as we go around upright - the stride.

THE SUBSTITUTE: Nature above all else is resourceful. It makes a living by devising exquisitely suited anatomical structures. Yet it's also a virtuoso at making do with what's available. The liver is the body's largest and most complex glandular organ, a chemical megaplex where digested proteins and fats are converted into dozens of life-sustaining substances. However, since before birth there's nothing to digest, the embryonic liver does short-term duty as a proxy, a temp, churning out red blood cells until corpuscle-producing marrow can be formed.

The respiratory system first arises during the fourth week as an inconspicuous groove at the rear of the facial region. From there, it grows downward, branching 23 times on both sides of the body, forming so called "long buds.” At 4 weeks the first buds appear and the lung starts developing. The tube for swallowing food (esophagus) separates from the breathing (laryngotracheal) tube. At 6 weeks secondary branches appear inside of the lung. These buds continue to branch after birth, eventually connecting 300 million tiny air bubbles - alveoli. If all your alveoli were spread out flat, they would cover a tennis court.

GENDER: As scientific nomenclature, the term seems almost comical - a sexual disorder among twentyish slackers who wear black and affect a studied ennui, or else men over 70. But in biology, indifference doesn't mean boredom: it means unmediated sameness. Up until 5 weeks after fertilization, the embryonic sex organs in boys and girls are identical. Genetically, sex has been assigned, but under a microscope the genital regions are indistinguishable - they haven't yet "differentiated" into male and female. When they do, in the third month, the initial protrusion that develops is shaped and molded in such a way that it can be become either a penis or a clitoris, with characteristics familiar to both.

TINY MINDS: "The brain," Emily Dickinson wrote, “is just the weight of God." By the seventh week, nerve cells in the brain have begun to touch via projecting molecules. Some start to form primitive pathways. The rate of production is stunning: 100,000 new cells a minute. At birth, the brain will consist of about 10 million densely entwined nerve cells - some 100 billion cellular units in all. Though single cells measure only one one-hundredth of a millimeter across, each is connected to thousands of others by finger-like branches along which it can send and relay signals - perhaps 10 trillion connections in all. Connectivity, not mass, accounts for the brain's exceptional abilities. "It has been estimated," Rugh and Shettles wrote, "that all 2 billion of the specific nerve cells which make any individual educable are located in the outer covering of his brain, its cortex, and that these 2 billion cells could be stored in a thimble."

KNITTING THE BODY TOGETHER: By 6 months, the bony skeleton is fully assembled but not yet connected at the joints. Like the soft wires pressed into the clay of a Gumby figure, the bones support the fetus structurally, allowing twisting and motion. But like a Gumby, it has no mechanical ability - its parts bend, but they can't bend and unbend at will. Your child will need another year at least before the ball-and-socket, hinge, and sliding mechanisms of the hip, knee, and ankle can deliver enough strength and flexibility to permit toddling. In adulthood, the human skeleton is an elegant moving framework, a torqued, white Eiffel Tower, levered and double-levered so that (theoretically) the same individual can pirouette, squat-lift a bag of cement, race the 100-meter dash, and perform surgery.

The skull is also synthesized modularly - a helmet molded from 4 partially calcified plates. In utero, the plates are loosely knit rather than fused. This allows the head to taper during delivery, and parents have learned not to be alarmed if their baby's head appears at first elongated or misshapen, or, later, when they discover a soft spot (a "fontanel", French for "little fountain") on the top of the skull. The human brain triples in mass during the first year, and the skull's flexible assembly both protects it and allows to grow. The skull bones interlock and fuse permanently at about 18 months. Meanwhile, the jawbone continues to protrude, undergoing a final spurt in late adolescence, when parents notice with confused emotions that their children suddenly "look like adults".