cell. Some parties contend that embryos are not human persons, believing that the life of Homo sapiens only begins when the heartbeat develops, which is during the fifth week of pregnancy, or when the brain begins developing activity, which has been detected at 54 days after conception. Efficiency In vitro fertilization (IVF) generates large numbers of unused embryos (e.g. 70,000 in Australia alone). Many of these thousands of IVF embryos are slated for destruction. Using them for scientific research uses a resource that would otherwise be wasted. While the destruction of human embryos is required to establish a stem cell line, no new embryos have to be destroyed to work with existing stem cell lines. It would be wasteful not to continue to make use of these cell lines as a resource. Superiority This is usually presented as a counter-argument to using adult stem cells, as an alternative that does not involve embryonic destruction. Embryonic stem cells make up a significant proportion of a developing embryo, while adult stem cells exist as minor populations within a mature individual (e.g. in every 1,000 cells of the bone marrow, only one will be a usable stem cell). Thus, embryonic stem cells are likely to be easier to isolate and grow ex vivo than adult stem cells. Embryonic stem cells divide more rapidly than adult stem cells, potentially making it easier to generate large numbers of cells for therapeutic means. In contrast, adult stem cell might not divide fast enough to offer immediate treatment. Embryonic stem cells have greater plasticity, potentially allowing them to treat a wider range of diseases. Adult stem cells from the patient's own body might not be effective in treatment of genetic disorders. Allogeneic embryonic stem cell transplantation (i.e. from a healthy donor) may be more practical in these cases than gene therapy of a patient's own cell. DNA abnormalities found in adult stem cells that are caused by toxins and sunlight may make them poorly suited for treatment. Embryonic stem cells have been shown to be effective in treating heart damage in mice. Embryonic stem cells have the potential to cure chronic and degenerative diseases which current medicine has been unable to effectively treat. Individuality Before the primitive streak is formed when the embryo attaches to the uterus around 14 days after fertilization, two fertilized eggs can combine by fusing together and develop into one person (a tetragametic