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I am trying to understand the formation of embryonic disc for human and chick so the following question is related to here about formation of embryonic disc.

I had thought that the amniotic sac forms only from embryonic disc in humans, particularly of lateral mesoderm and ectoderm of embryonic disc. However, my thought seems to be wrong as my professor says that amnion form only for birds from embryonic disc, but from many different parts besides of embryonic disc for human.

What, besides the embryonic disc, forms the human amniotic sac?

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In humans, the amnion (amniotic sac) persists from the primitive amniotic cavity1. One side of this is formed from the cytoblast (a prismatic epithelium) and the plasmodioblast. Together these two layers are the ectoplacenta or chorion. They are also referred to as Rauber's layer. These replace the lining epithelium of the uterus, whereupon internal cells undergo atrophy to create the amniotic sac.2

The other side of the amniotic sac is formed of the epiblast/ectoderm (internally) and the hypoblast (externally). Within the epiblast the other layer is comprised of prismatic cells whereas the inner layer are flattened (the hypoblast/entoderm). This double layer forms the bilaminar blastodermic membrane.2

I'd really reccomend that you borrow the referenced books from your university library as I found the diagrams much easier to understand than the text.


1 Gray, Henry. "Embryology: Formation of Membranes." Ed. Robert Howden. Anatomy Descriptive and Surgical. Ed. T. P. Pick. 15th ed. London: Chancellor, 1994. 90. Print. Colloquially Gray's Anatomy

2 Gray, Henry. "Embryology: The Ovum." Anatomy Descriptive and Surgical. Ed. T. P. Pick and Robert Howden. 15th ed. London: Chancellor, 1994. 82-83. Print. Colloquially Gray's Anatomy

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