From yesterday’s hearing:

University of Massachusetts ethicist Marjorie Clay said stem cells created for research and treatment are not really embryos, since they cannot grow alone.
“A fetus will not happen unless that clump of cells is implanted in a uterus,” Clay said to the legislative panel, adding that excess embryos from fertility procedures are discarded regularly. “If we thought they were really persons, would they pitch them?”

This Ramesh piece from 2001 is worth rereading.