Begining of Human Life

Given the firestorm of controversy surrounding such subjects as stem cell research and abortion, I was wondering what the Talmudic or Halachic basis is for the begining of human life? Growing up I was taught that according to Jewish law, until a fetus is actually born, they are not yet considered a human being. Many Christians and other religions seem to define the begining of human life at the time of conception. Even scientists disagree as to when a human being comes into existence from the mass of cells that it is its first formed from. I would appreciate any insights form the rabbis on this question.

Shalom U'veracha, This is a huge question and much has been written about this issue. The Gemara clearly states (Yevamot 69b) that the fetus is considered like water until forty days after fertilization. Some explain this only in relation to a fetus ex-vivo, i.e. outside the body, such as a fetus in the laboratory, but when dealing with a fetus in the uterus we have to relate to it as a living being even before 40 days. In light of this it is clear that stem cell research and use would be allowed. Kol Tuv