According to Stith, "This widespread vision of the embryo and fetus as 'under construction' is the key to understanding why good people may find pro-life arguments to be absurd or otherwise non-rational, e.g., religious, particularly with regard to embryonic stem cell research."
Stith gives this example:
Just think of something being constructed (fabricated, assembled, composed, sculpted – in short, made), such as a house, or a scholarly article – or take a car on an assembly line. When is a car first there? At what point in the assembly line would we first say, "There's a car"? Some of us would no doubt go with appearance, saying that there is a car as soon as the body is fairly complete (in analogy to the fetus at 10 weeks or so). I suppose that most of us would look for something functional. We would say that there is a car only after a motor is in place (in analogy to quickening). Others might wait for the wheels (in analogy to viability) or even the windshield wipers (so that it's viable even in the rain). And a few might say, "It's not a car until it rolls out onto the street" (in analogy to birth). There would be many differing opinions.Thinking of the embryo in this way is scientifically mistaken. It is not a correct understanding of the nature of living things, which are not constructed, but rather develop. We know this as a matter of fact.
However, one thing upon which we'll probably all agree is this: Nobody is going to say that the car is there at the very beginning of the assembly line, when the first screw or rivet is put in or when two pieces of metal are first welded together. (You can see how little I know about car manufacturing.) Two pieces of metal fastened together don't match up to anybody's idea of a car.
I think that this is exactly the way that many people see the embryo, like the car-to-be at the very beginning of the construction process. In the first stages of construction you don't have a house, you don't have a car, you don't have a human individual yet. You don't ever have what you're making when you've just started making it.
Scott Klusendorf puts it like this:
Embryos aren't constructed piece by piece from the outside; they develop themselves from within. That is to say, they do something no constructed thing could ever do: they direct their own internal growth and maturation, and this entails continuity of being. Unlike cars, developing embryos have no outside builder. They're all there just as soon as growth begins from within. In short, living organisms define and form themselves. An oak tree is the same entity that was once a shoot in the ground, years before it had branches and leaves.In other words, each one of us was once an embryo. Embryos are whole organisms from the beginning of their existence, fully programmed to develop themselves through the different stages of human life. Stith adds a helpful analogy:
Here is a non-biological example of development. Suppose that we are back in the pre-digital photo days and you have a Polaroid camera and you have taken a picture that you think is unique and valuable – let's say a picture of a jaguar darting out from a Mexican jungle. The jaguar has now disappeared, and so you are never going to get that picture again in your life, and you really care about it. (I am trying to make this example parallel to a human being, for we say that every human being is uniquely valuable.) You pull the tab out and as you are waiting for it to develop, I grab it away from you and rip it open, thus destroying it. When you get really angry at me, I just say blithely, "You're crazy. That was just a brown smudge. I cannot fathom why anyone would care about brown smudges." Wouldn't you think that I were the insane one? Your photo was already there. We just couldn't see it yet.You can find more about fetal development here, and Scott Klusendorf summarizes Stith's analysis here.