This scanning electron micrograph shows a human spermatozoon contacting a hamster egg (×13,200). The ability of a patient’s spermatozoa to penetrate hamster eggs is used in an assay to measure male fertility. In the assay, a number of hamster eggs are exposed to spermatozoa. The penetration of more than 10 to 25 percent of the eggs is considered to be normal. Fertilization, of course, does not occur.