卵黄嚢はhypoblast(胚盤葉下層)からできてくるんだよね。
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Embryonic Development of the Hypoblast:
The inner cell mass (ICM) segregates into a bilaminar embryonic disc (bilaminar blastoderm) which consists of two epithelial layers, each of a distinct lineage: the external (dorsal) epiblast and the internal (ventral) hypoblast. The hypoblast is a transient structure which contributes to the development of the extraembryonic mesoderm and yolk sac and plays a key role in signaling to establish axial patterning in the embryo itself.
