Editing Pregnancy (mammals) (section)

===Placental circulation system===
The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood, which in turn stimulate the production of capillaries. This is the first stage in the growth of the [[placenta]]. The inner cell mass of the blastocyst divides rapidly, forming two layers. The top layer becomes the embryo, and cells from there occupy the [[amniotic cavity]]. At the same time, the bottom layer forms a small sac (if the cells begin developing in an abnormal position, an ectopic gestation may also occur at this point).

Several days later, [[chorionic villi]] in the forming placenta anchor the implantation site to the uterus. A system of blood and [[blood vessel]]s now develops at the point of the newly forming placenta, growing near the implantation site. The small sac inside the blastocyst begins producing [[red blood cell]]s. For the next 24 hours, [[connective tissue]] develops between the developing placenta and the growing [[embryo]]. This later develops into the [[umbilical cord]].