The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth (sometimes incorrectly referred to as the maternal part of the placenta). Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.Mammalian placentas probably first evolved about 150 million to 200 million years ago. The protein syncytin, found in the outer barrier of the placenta (the syncytiotrophoblast) between mother and fetus, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient retrovirus: essentially a virus that helped pave the transition from egg-laying to live-birth.The word placenta  comes from the Latin word for a type of cake, from Greek πλακόεντα/πλακοῦντα plakóenta/plakoúnta, accusative of πλακόεις/πλακούς plakóeis/plakoús, "flat, slab-like", with reference to its round, flat appearance in humans. The classical plural is placentae, but the form placentas is more common in modern English.
Some Malays would bury the babys placenta with a pencil (if it is a boy) or a needle and thread (if it is a girl).In some cultures, the placenta is eaten, a practice known as placentophagy. In some eastern cultures, such as China, the dried placenta (ziheche 紫河车, literally "purple river car") is thought to be a healthful restorative and is sometimes used in preparations of traditional Chinese medicine and various health products. The practice of human placentophagy has become a more recent trend in western cultures and is not without controversy; its practice being considered cannibalism is debated. Some cultures have alternative uses for placenta that include the manufacturing of cosmetics, pharmaceuticals and food. Additional images
See also
Choriovitelline placenta
Caul
Zygote
Pregnancy in fish

References
External links

The placenta-specific proteome at the Human Protein Atlas
The Placenta, gynob.com, with quotes from Williams Obstetrics, 18th Edition, F. Gary Cunningham, M.D., Paul C. MacDonald, M.D., Norman F. Grant, M.D., Appleton & Lange, Publishers.