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.
There are multiple definitions for the term frontier and seldom are they in agreement. Frontier may be interpreted as a subjective state of mind or an objective quantitative measure. In essence, the definition of frontier means sparsely populated rural areas with a population density of six or fewer people per square mile. The frontier may be defined at the county level, by zip code or census tract and be dependent on the project being researched or funded.