Discussion
During the development of a mammalian embryo, "yolk sac" is formed by which one of the following?
Answer: B
The yolk sac is formed by the primitive endoderm (hypoblast).
In the mammalian blastocyst the inner cell mass separates into the epiblast and the hypoblast. Hypoblast cells spread out over the inner surface of the blastocyst cavity as extra-embryonic endoderm and line it, forming the primary and then the secondary yolk sac, which is later covered by extra-embryonic mesoderm.
The syncytiotrophoblast derives from the trophectoderm and invades the uterine wall to form the placenta. The amniotic ectoderm and the epiblast belong to the amnion and the embryo proper, not to the yolk sac.
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