Final answer:
Embryologic development of the face occurs primarily between the fourth and eighth weeks after fertilization, with major facial features forming around the sixth to seventh weeks and becoming distinctly human by the eighth week.
Step-by-step explanation:
The embryologic development of the face occurs mainly between the fourth and eighth weeks after fertilization. At around six weeks, features such as nostrils, outer ears, and lenses begin to form, and by seven weeks, the facial structure becomes more complex. The eighth week marks a significant stage where the face starts to look human, with external ears formed, fetal heartbeat detectable by ultrasound, and the head being nearly as large as the rest of the embryo's body. This period is of utmost importance as it is during organogenesis, where all essential organs begin to form. Harmful environmental exposures during this time, such as toxins, alcohol or radiation, could lead to severe impacts on development.