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

[EX Q]: What begins approximately six weeks after fertilization in an embryo?
[EX A]: ossification

[EX 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?
[EX A]: yolk sac

[EX Q]: Momentum is directly related to both mass and?
[EX A]:
velocity