Scientists are now starting to develop lab grown human embryos past the 10 week fertilization period. These human embryo models are being made from stem cells that mimic the development of human embryos. The scientific goal is to be able to study human development past the first week or so which is usually when the embryo would need to be implanted in a human to continue its development. These models have caused both excitement and concern as they may be used to mimic babies. This raises the issue on whether or not to to regulate what scientists can and cannot do with the embryos. For years scientists have been able to study donated embryos which helped with learning about the blastocyst which is needed to form the placenta inside of the uterus, but the most interesting part of the embryo is from day 7-35 when the actual development occurs which is usually after implantation. These embryo replicas contain most cells needed but not the trophectoderm which would be needed to develop the placenta which would then be needed for further development of the embryo. Since embryos can only be implanted if they have from 1-64 cells, these embryos would never successfully implant as they are late stage which have well beyond the 64 cells.