instruction:
Given a scientific question, generate a correct answer to it.
question:
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 this?
answer:
yolk sac

question:
What are the two main minerals found in ocean water?
answer:
sodium, chloride

question:
Muscles are composed mostly of cells called what?
answer:
muscle fibers