Ava is five weeks pregnant, and her fetus is beginning to develop three layers that will form the essential human functions. What will the ectoderm layer become over the course of Ava's pregnancy?
1. heart, circulatory system, bones, muscles, and reproductive system
2. skin, nervous systems, connective tissue, and eyes
3. lungs, intestines, and bladder
4. hair, fingernails, and eyebrows