Given a scientific question, generate a correct answer to it.

[Q]: What is the name for animals that have a backbone?
[A]: vertebrates

[Q]: 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?
[A]: yolk sac

[Q]: What is caused by differences in density at the top and bottom of the ocean?
[A]:
deep currents