The new human life began through simple cell division as it floated down the fallopian tube of the mother. At the beginning, the cell division was rather simple. One division, and then another and then another. Two, four, eight, sixteen tiny cells formed. The new human life, called a zygote at this point, made its way for about three days down the tube toward the womb being nourished all the while by secretions from the mucous membrane lining the tube.

When the new life, now called a morula, reached the uterus it looked like a little mulberry composed of 60 or more cells, but it was too tiny to be seen easily with the naked eye. The number of cells kept increasing rapidly, however, and finally the blastocyst, a new name given this little life, collapsed in on itself, resembling a punched-in, deflated soccer ball. The blastocyst floated for a short time in the womb, nurtured now by secretions from the lining of the womb. It finally settled in and implanted itself in the upper portion of the womb.

At the very beginning of the cell divisions of this new life, any one of those cells could have developed into a full human being. Called toti-potential cells, they each had the potential to become any part of the human person—or another human being altogether. After the initial cell division, the cells began to differentiate themselves into the various types of specialized cells in the body. This process was very complex and unfathomably intricate. Each cell type developed in the right place of the mass of cells and at the right time. The early differentiated cells were somehow able to “read” their position in relation to other developing cells. Daughter cells then differentiated further, remembering their position so that they would develop into brain cells, or skin cells, or hair cells, or blood cells. These differentiated cells all came to be ordered into an ever more complex organism until eventually they formed all the cells of the young child.

As the blastocyst, or zygote, drove into the lining of the womb, it divided into two parts. One half attached to the wall of the womb and became the placenta which enabled the child to receive food and liquids from the mother and to pass off waste. After the zygote actually burrowed beneath the lining of the womb, it developed a kind of root system pushing into the uterine lining. These chorionic nuclei secreted a chemical which prevented other eggs in the mother's ovaries from maturing. They also helped to secure the child in the mother's womb. The other half of the division of cells developed into the embryonic disk, an early phase of the child itself. The umbilical cord developed finally, connected to the placental villi which drew nutrients, oxygen, and fluid from the mother.

In the fifth week of its life the embryo was about the size of an apple seed. Already all the major body organs and systems were formed, although they were not yet developed. Two folds of skin, which would become ears, appeared at this time on either side of the head. In the sixth week the neural tube, which connects the brain and the spinal cord, closed.

In the seventh week the embryo was about the size of a small bean. Dark spots appeared where the eyes would be. Openings for the nostrils began to form, and little buds which would become the arms and legs stuck out from the tiny body which looked a bit like a tadpole. It was in this week that the heart divided into two chambers and began to beat about 150 beats a minute—twice as fast as an adult. Muscle fibers began to grow and enough nerve cells developed that the fetus would pull away from stimuli.

In the eighth week tiny fingers and toes began to become distinct. The teeth, palate, and larynx formed, the ears took shape, and tiny veins were visible through the baby's thin skin. At this point some women do not even know they are pregnant. This woman, however, knew she was pregnant.

The baby went through twenty-eight more weeks of spectacular growth as it floated in the amniotic fluid in the uterus. One day the mother felt it stir in her womb. It turned and pulled up its arms and legs. It would punch out its knee. Sometimes he would suck his thumb. As the time of his birth grew near, he settled into a comfortable position in the uterus with the head down. The time came. When the child emerged, he was a marvel to behold. The intricately sculpted ears, the little clenched fist, the tiny toes. And they named him Jesus.

Dr. John Haas is president of the National Catholic Bioethics Center in Boston.