Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: While the egg is developing, other changes are taking place in the uterus. It develops a thick lining that is full of tiny blood vessels. The lining prepares the uterus to receive a fertilized egg if fertilization actually takes place. Answer: tiny blood vessels
A:
While the egg is developing, other changes are taking place in the uterus. it develops a thick lining that is full of what?