Slide 8 of 49

Human Embryology & Teratology: "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... "The ill-defined and inaccurate term pre-embryo, which includes the embryonic disc, is said either to end with the appearance of the primitive streak or ... to include neurulation. The term is not used in this book." (p. 55)."

O'Rahilly, R. and F. Muller. 1996. Human Embryology & Teratology, Wiley-Liss, New York, pp. 5-55.