Extraembryonic Layers MCQ - Practice Questions with Answers

• These membranes are formed outside the embryo from the trophoblast and embryonic layers only in amniotes (reptiles, birds and mammals) and perform specific functions.
• There are four extraembryonic membranes:
  • Amnion
  • Yolk sac
  • Allantois
  • Chorion

Amnion:

• At the beginning of the second week, the cells of the inner cell mass form into a two-layered disc of embryonic cells, and space—the amniotic cavity—opens up between it and the trophoblast. 
• Cells from the upper layer of the disc (the epiblast) extend around the amniotic cavity, creating a membranous sac that forms into the amnion by the end of the second week.
• The amnion fills with amniotic fluid and eventually grows to surround the embryo.  

Yolk sac:

• On the ventral side of the embryonic disc, opposite the amnion, cells in the lower layer of the embryonic disk (the hypoblast) extend into the blastocyst cavity and form a yolk sac.
• The yolk sac supplies some nutrients absorbed from the trophoblast and also provides primitive blood circulation to the developing embryo for the second and third week of development. 
• When the placenta takes over nourishing the embryo at approximately week 4, the yolk sac has been greatly reduced in size and its main function is to serve as a source of blood cells and germ cells.

Allantois:

• During week 3, a finger-like out pocketing of the yolk sac develops into the allantois, a primitive excretory duct of the embryo that will become part of the urinary bladder.
• Together, the stalks of the yolk sac and allantois establish the outer structure of the umbilical cord.

Chorion:

• The last of the extraembryonic membranes is the chorion, which is the one membrane that surrounds all others.
• It takes part in the formation of the placenta.