Embryological origins of the human individual.

The human fertilized egg is not an embryo, but, more accurately, a conceptus, which contains all the information required to establish both the embryo and extraembryonic supporting tissues. The fertilized eggs of placental mammals, including humans, are entirely dependent upon the female's uterine environment for development to birth. At least half, possibly more, fertilized eggs, or potential lives, do not survive to birth, the greatest loss thought to occur during preimplantation and implantation. Which conceptuses will be lost and which will progress to birth cannot be predicted. The. preimplantation conceptus exhibits extreme developmental lability. Importantly, twinning can occur throughout the preimplantation and implantation phases, and thus, a single human individual has not emerged from the conceptus during this time period. Once the primitive streak is complete during early postimplantation development, identical twinning no longer occurs and the individual emerges. Thus, emergence of the human individual is a process. No single event thus far known is more important than any other prior to formation of the primitive streak. Formation of the streak is a defining moment in the origin of the individual. All further organization of the fetus occurs around this midline. Thus, by 14 days in the human, the body plan for life is established.