The result of hens and roosters mating, fertile eggs contain both male and female genetic material. A fertile egg begins the first step of embryonic development (a “blastoderm” appears as a whitish spot on the yolk), but does not develop any further without incubation. Because mating is not necessary for hens to lay eggs, roosters are rarely housed with hens on commercial egg farms, and nearly all eggs found in grocery stores are infertile (female genetic material only).
Eating fertile eggs is a trend popular among Paleo and raw food followers who promote fertile eggs as being more nutritious than infertile eggs. However, there are actually no detectable chemical differences between fertile and infertile eggs, nor is there any scientific evidence that one is nutritionally superior to the other.
There is, however, some evidence that fertile eggs may spoil sooner than infertile eggs.