An essay by Dr. Robert George, written for and published at Pontifications:

Now a supporter of embryo-killing for biomedical research might concede that a human embryo is a human being, yet deny that human beings in the early stages of their development are due full moral respect such that they may not be destroyed to benefit more fully developed human beings who are suffering from afflictions.

But to deny that embryonic human beings deserve full respect, one must suppose that not every whole living human being deserves full respect. And to do that, one must hold that those human beings who deserve full respect deserve it not in virtue of the kind of entity they are, but, rather, in virtue of some acquired characteristic that some human beings (or human beings at some stages) have and others do not have, and which some human beings have in greater degree than others.