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.
Noun. 1. boundary-the line or plane indicating the limit or extent of something. bounds, bound. hairline-the natural margin formed by hair on the head.frontier-an international boundary or the area (often fortified) immediately inside the boundary.. boundary-the line or plane indicating the limit or extent of something. bounds, bound. hairline-the natural margin formed by hair on the head. frontier-an international boundary or the area (often fortified) immediately inside the boundary.