But the moral controversy surrounding embryonic stem cells -- the fact that these cells are derived from five-day old human embryos -- has encouraged research into other types of stem cells.
不同种类的细胞。 但是之所以胚胎干细胞 在道德上备受非议是 因为这些细胞来源于 5天大的人类胚胎-- 正是因为这种争议才激励人们研究 其他类型的干细胞。