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Molecular membranes , also called embryonic envelopes , fruit envelopes or germ layers , surround the developing embryo or fetus in the egg of some arthropods and in the egg or uterus of amniotic land vertebrates, including humans. It is tissue formed by the embryo itself.

In insects and scorpions, as well as birds , reptiles and mammals, a distinction is made between two membranes, the amnion and the serosa. The amnion is the inner membrane and the serosa is the outer. The amnion and serosa of the arthropods are not homologous to the membranes of the amniotes. The serosa is at Amniota normally chorionic called in insects, however, is the chorion no amniotic membrane, but a (secondary) Case of unfertilized ice formed even in the ovary of the mother animal, and either homologous or analogous to the serosa of the Amniota.

Embryonic envelopes of the amniotes

An amniotic egg with the embryo (a) and the four extraembryonic membranes are shown schematically: b = yolk sac, c = allantois, d = amnion, e = serosa (chorion)

The liquid in the amnion is Amniota amniotic called, and amnion and serosa form as Fetalmembranen advanced fetal stage along the amniotic sac .

In most amniotes, the amnion and serosa are formed by joint unfolding of the outer ( ectoderm ) and middle cotyledons ( mesoderm ) with a subsequent union “above” the dorsal side of the embryo. In these cases one speaks of a fold amnion . The space between the amnion and the serosa is called extraembryonic coelom and is lined with mesodermal cells. However, some mammals, including humans, do not form a fold amnion, but a fissure amnion (schizamnion), i.e. H. In the upper part of the inner cell mass of the blastocyst (epiblast) a cavity is formed, the bounding cell layers of which differentiate into the amnion (“split off”). In the "real" mammals (Theria) part of the serosa is involved in the formation of the placenta , which is particularly strong in the placental mammals (Eutheria). However, not only embryonic tissue is involved in the placenta, but also maternal tissue with part of the uterine lining (decidua).

The amnion and serosa are also known as extraembryonic membranes . Two other such membranes are the yolk sac and the allantois (embryonic urinary bladder). Both are traversed by a blood vessel system and protrude into the extraembryonic coelom. The yolk sac is connected to the embryonic midgut via the yolk stalk . The allantois is associated with the embryonic rectum. In addition to excretion, the allantoic membrane in sauropsids (" reptiles " and birds ) takes over gas exchange in the egg. It gradually fuses with the serosa to form the chorioallantois . While the embryonic part of the placenta in marsupials is formed by fusing parts of the yolk sac membrane with the serosa (yolk sac placenta), in placental mammals it is formed by fusing parts of the allantoic membrane with the serosa (allantochorion placenta). The connection between the allantois or placenta and the fetal intestine is called the umbilical cord in placental mammals .


  • Hartmut Greven: Reproduction and Development. P. 167-182 in: Wilfried Westheide, Gunde Rieger (Ed.): Special Zoology. Part 2: vertebrates or skulls. 2nd edition, Spektrum Akademischer Verlag, Heidelberg 2010, ISBN 978-3-8274-2039-8

Individual evidence

  1. Embryonalhüllen in the Lexicon of Biology.
  2. ^ Anne-Katrin Eggert, Josef K. Müller, Ernst Anton Wimmer, Dieter Zissler: Reproduction and Development. P. 363–459 in: K. Dettner, Werner Peters (Ed.): Textbook of Entomology. 2nd edition, Spektrum / Elsevier, Munich 2003, ISBN 3-8274-1102-5 , p. 369.
  3. Kirsten Ferner, Andrea Mess: Evolution and development of fetal membranes and placentation in amniote vertebrates. Respiratory Physiology & Neurobiology. Vol. 178, No. 1, 2011, pp. 39-50, doi: 10.1016 / j . or 2011.03.029 (alternative full-text access : ResearchGate ).