Human-animal chimeras are taking over the barnyard. A year after a research team announced they had created the first animal-human chimera by way of pig embryos containing human cells, they've done it again, the Guardian reports. This time it's sheep embryos with human cells. The goal is to improve the availability and success rates of organ transplants by growing human organs inside animals. “Even today the best matched organs, except if they come from identical twins, don’t last very long because with time the immune system continuously is attacking them,” Dr. Pablo Ross says. According to the Telegraph, Ross' team grew the embryos inside a sheep for three weeks. He says sheep have a number of advantages over pigs, including that their embryos are easier to produce via IVF. A professor of animal biotechnology calls it "an important step forward," but Ross' team is a long way off from actually growing human organs inside an animal. The embryos his team created contained about one human cell for every 10,000 sheep cells; he thinks it needs to be closer to one human cell for every 100 sheep cells to be successful. There's also the ethical quandary of potentially producing animals with human-like intelligence. "If our results indicate that the human cells all go to the brain of the animal, then we may never carry this forward,” Ross says. Another team member, Dr. Hiro Nakauchi, says that's unlikely: “The contribution of human cells so far is very small. It’s nothing like a pig with a human face or human brain." Ross' team announced their sheep-human embryos at this week's meeting of the American Association for the Advancement of Science in Texas.