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.
Re: Border vs Frontier vs Boundary. A simple rule: Countries have borders. Boundary is for other separations between geographical areas. The only frontier this American would talk about would be historical between settled land and the untamed wilderness.he point where you cross from one country to another is usually called the border. In British English it can also be called the frontier, but this is often in a context of wildness, danger and uncertainty: The rebels control the frontier and the surrounding area.