A team of U.S. and Chinese scientists have produced a series of human-monkey hybrid embryos in a study released Thursday that is raising critical bioethical questions about its experiments and their implications. In the study funded by the National Natural Science Foundation of China and published in the journal Cell, scientists took six-day-old macaque monkey embryos and injected 25 human cells into each of them. The type of human cells used are extended pluripotent stem cells, which can help develop both embryonic and extra-embryonic tissues, according to a Cell news release. One day following the injections, human cells were found alive in 132 of the hybrid embryos. Ten days later, 103 of them were still alive and growing. However, survival rates started dropping to the point that by day 19, only three of them were alive. The embryos that did survive, however, had a high percentage of human cells in their mix. The researchers conducted this study as part of an effort to discover new methods of creating organs for those in need of transplants.