We could compare the manufacture of a bicycle with the development of the human fetus.
A web site describes an electric, 30-40 km range bicycle using a web-rechargeable battery. In manufacturing this vehicle engineers and workers have worked together and assembled the various parts, such as wheels, seat, handlebars, motor, battery, etc. And they have built a bicycle. They can add ornaments and change colors and shapes.
In a human being, as from conception nobody can add anything from outside, nor can the color of the eyes or the intellectual ability be modified.
The egg is the union of two cells—the ovum from the mother and the spermatozoon from the father. These two cells develop quickly until, by the end of the third month, they already have a human form and it is called a fetus. The different systems develop gradually, including the digestive, the respiratory, the circulatory system, among others. The reproductive system will finish developing by puberty and adolescence. Neither the father nor the mother are involved in its formation.
The egg has all the new being’s features already imprinted on its genes, to be further developed over the nine months till birth. It is a new being with a life of its own which, even though it is in the mother’s womb, is not part of the mother’s body.
There lies the difference between the manufacture of a bicycle—or a car or a house—and the development of a human being. The American sociologist Dalton Conley wrote in the New York Times in 2005 that, “most Americans see a fetus as an individual under construction.” This is an obvious mistake: neither the egg nor the fetus are manufactured by external agents, but rather develop on their own.
Comparing the manufacture of a bicycle to a baby’s development might be a nice image, but it is altogether wrong. Wheels do not form the bicycle alone. Only once all parts have been assembled can we say that we have the vehicle. If during the (unfinished) manufacturing process we realize that we do not like the outcome and it gets discarded, we will not be throwing away a bicycle but a number of parts. This is not the case for human beings. If by external means the fetus’ head is crushed inside the mother’s womb and the little body is removed in pieces and thrown into a garbage bag, it is not a few cells that have been thrown away, but an independent being under development, that is, a human person.