Why are some of embryonic stem cells localized out of the embryo?

Pluripotent embryonic stem cells can produce any of the cell types of the body, although not the extraembryonic cells that go on to form structures such as the placenta. A group of pluripotent embryonic stem cells reside in the extraembryonic tissues such as allantois and yolk sac in the early developmental stages. The cells that enter the allantois do not return to the embryo, whereas other cells migrate in the embryo to reach the developing gonads. By not migrating into the embryo and staying in the allantois arrests the normal program of change of their character with time and so enables these stem cells to live without differentiating. It seems that in the early developmental stages, a group of pluripotent embryonic stem cells each of which can give rise to an embryo are set aside out of the embryo for a possible use in the future. [J Exp Integr Med 2012; 2(4): 373-375]