Sperm penetration tests check to see whether a man's sperm can move through cervical mucus and the fallopian tubes to join with (fertilize) an egg. This test is usually done when a couple is having trouble becoming pregnant (infertility). There are different sperm penetration tests.
The idea that a boundary is shifting can be applied to different areas. A shifting frontier of science would be referring to the fact that science is always changing and growing, and always with new areas. In wartime, a shifting frontier would be an international border which is constantly being renegotiated.
Define frontier: a border between two countries; a stronghold on a frontier â frontier in a sentence a border between two countries; a stronghold on a frontier; a region that forms the margin of settled or developed territoryâ¦ See the full definition since 1828