My favorite is this one:
“Fertilization is an important landmark because, under ordinary circumstances, a new, genetically distinct human organism is thereby formed… Fertilization is the procession of events that begins when a spermatozoon makes contact with a secondary oocyte or its investments… The zygote … is a unicellular embryo…”
Human Embryology & Teratology 1996 pg. 5-55.
And, of course, I love the picture at the top of the post; click here to see it.