the two major lineages that make up the living amniota are
synapsids sauropsids
title: Amnion text: 'lamb'. it is cognate with the English verb 'yean', bring forth young (usually lambs). The amnion is a feature of the vertebrate clade Amniota, which includes reptiles, birds, and mammals. Amphibians and fish are not amniotes and thus lack the amnion. The amnion stems from the extra-embryonic somatic mesoderm on the outer side and the extra-embryonic ectoderm or trophoblast on the inner side. In the human embryo, the earliest stages of the formation of the amnion have not been observed; in the youngest embryo which has been studied the amnion was already present as a closed sac, and appears in title: Amniote text: as required of the amphibians. From this point the amniotes spread around the globe, eventually to become the dominant land vertebrates. Very early in their evolutionary history, basal amniotes diverged into two main lines, the synapsids and the sauropsids, both of which persist into the modern era. The oldest known fossil synapsid is "Protoclepsydrops" from about 312 million years ago, while the oldest known sauropsid is probably "Paleothyris", in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306–312 million years ago). Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion, title: Amnion text: Amnion The amnion is a membrane that closely covers the embryo when first formed. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo or fetus. The amnion, along with the chorion, the yolk sac and the allantois form a protective sac around the embryo. In birds, reptiles and monotremes, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in a uterus. The term amnion is from Ancient Greek ἀμνίον 'little lamb', diminutive of ἀμνός title: Amniotic sac text: Amniotic sac The amniotic sac, commonly called the bag of waters, sometimes the membranes, is the sac in which the fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth. The inner of these fetal membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the fetus. The outer membrane, the chorion, contains the amnion and is part of the placenta. On the outer side, the amniotic sac is connected to the yolk sac, the allantois and, via the umbilical cord, to title: Amnion text: amnios attaining the inner surface of the placenta. The amnios consists of two layers:one is the granular layer, continued upon the inner or foetal surface of the allantois, and thence upon the umbilical cord; the other is the smooth outer layer, continued upon the outer or chorional surface of the allantois, and thence upon the inner surface of the chorion." Amnion The amnion is a membrane that closely covers the embryo when first formed. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the