Final answer:
During weeks 17-20 of fetal development, the fetus experiences the advancement of touch and hearing, early formation of alveoli, ossification, muscle development, blood cell production in the bone marrow, and the development of vernix on the skin.
Step-by-step explanation:
Between weeks 17 to 20 of fetal development, a fetus undergoes several significant changes. The senses of touch and hearing continue to develop, allowing the fetus to start responding to external stimuli. The alveoli, which are critical for gas exchange in the lungs, begin their initial formation. Ossification, or the process of bone formation, is well underway, leading to considerable muscle development and strengthening of the skeletal structure. Furthermore, the bone marrow becomes active in producing blood cells, which is a pivotal step in the fetus's ability to supply oxygen throughout its body. In addition to these developments, a protective, waxy substance known as vernix starts to cover the fetus's skin, shielding it within the amniotic fluid.