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.
Shifting cultivation Shifting cultivation is an agricultural system in which a person uses a piece of land, only to abandon or alter the initial use a short time later. This system often involves clearing of a piece of land followed by several years of wood harvesting or farming until the soil loses fertility.