So, let me get this straight. You are asking anti-abortion folks to make the choice between saving 40 lives or saving 1, right?
OK - on a practical level I think most people, (including myself) would save the infant, simply because a petri dish does not confront us with a face. I assume in a crisis situation, (especially one where we aren't physically able to handle both an infant and a small dish simultaneaously), that we don't have time to ponder the 40 "invisble" lives while presnted with the one very visible life. Put frankly, I don't think the lives in the dish would cross our mind.
Maybe you can reconstruct the scenario so that we have time to contemplate both choices.
But let's explore further. Supposing we have in fact been thinking about those 40 lives in the dish for some time before the fire breaks out. Supposing we have, in fact, had them very much on our mind leading up to the very second the crisis hit. Supposing even more, that the infant has been present as well, not so much the focus of our thoughts but occupying our attention equally. In that way, both the embryo's and the infant are on the same mental as well as moral plane. So, who do we choose then?
Let's make it even better. Suppose the infant is ours, but we have become infertile, and the forty embryos represent our only other hope for more children. I mean, let's really make it "Sophie's Choice", eh? I suppose someone could justify in their mind taking the infant because "a child in the hand...". I think I would be inclined to take the child I actually have versus the children I might only potential be able to bare. (Note, I am not saying the embyos are "potential life", only that they, as embryo's, only have a chance of developing to birth). In fact, those forty embryos probably only represent one more chance of producing one more child, based on the ratios of emrbyos to actually produced children that usually occur in infertility programs. So, the choice of the born child is easy.
I suppose we could also look at it from the emotional perspective that typically is taken by abortion advocates. The born child certainly, currently, has the "better" life, and therefore should be the one saved.
And I suppose we could look at it from another emotional perspective - who is dying going to "hurt" more. Embryo's despite what some may claim, don't "experience" death like born people do, especially the kind of horrifying death which would occur for the infant. So, let the one's who are unaware of their existence die and save the one who is self aware.
And finally, we could look at it from the simple standpoint that we don't consider the embryos valuable yet. I certainly can't envision embryo's in a dish having souls yet and therefor can't imagine God placing more value on them than the born infant. So, save the one that God has already invested a soul in.
For the first and last reasons, I personally would save the infant. And shame on you for putting people through such moral angst.
Of course, you avoided presenting plan "C" - "You can save both the dish and the infant at the loss of your own life"