A recent change in federal regulations enables government-funded researchers to experiment using embryonic tissue. How is an embryo different from a fetus?
After the 10th week of pregnancy, a human embryo is more commonly referred to as a foetus (which, incidentally, is the same thing as a fetus) to signify a new phase of development. At 10 weeks, the embryo-turned-fetus has just completed its most rapid period of cellular differentiation, a process that formed the beginnings of the most basic human structures. Roughly 4 centimeters long, the new fetus lacks working lungs and is many weeks away from being viable outside the womb. Still, with the hint of fingers, toes, and ears, it is now clearly recognizable as human.