A hen and a rooster must mate before any of her eggs will be fertilized. Hens in flocks without roosters still lay eggs, however none of the eggs will be fertile. Eggs can only be confirmed as fertilized if they either are cracked open, or if they produce a chick after incubation.
The little white stringy blob in unfertilized hen's eggs is not an embryonic chick. It's a tightly twisted egg white called chalazae that keeps the yolk safely in the center of the egg.
All in a Day's Work
After a rooster and hen mate, sperm from the rooster is stored in the hen's oviduct, allowing her to lay fertilized eggs for about two weeks. After two weeks she will need to mate with the rooster again to continue producing fertilized eggs. If she does not, she will lay eggs daily, however these eggs will not be fertilized.
Depending on whether he has had a "long day at the office," a rooster can inseminate a hen with 100 million to 5 billion sperm in one mating. The higher amounts come early in the day, with lesser ejaculates occurring after several matings. If he has little to no competition, and plenty of hens to choose from, a rooster can mate anywhere from 10 to 30 times in a day.
The rooster's sperm is stored in the hen, in a small cavity called the infundibulum. An ovum, or bare yolk, emerges from the hen's ovaries and drops into the infundibulum. There it is combined with a teeny bit of the rooster's sperm. Then the yolk is moved along the hen's oviduct: This is where it begins to form the layers that turn it into an egg.
At the end of the route the hen's uterus contracts, pushing the egg through the vent in a movement called oviposition; we just call it "laying an egg." The whole process from start to finish takes about 24 hours. A new yolk is produced half an hour after the most recent egg has been laid.
Blood Spots, Blastos and Bull's-eyes
Sometimes you'll crack an egg into the pan only to be horrified to see a small red blood clot invading your breakfast. If you've purchased your eggs from the store, this little blood spot is not -- nor was it ever -- an embryonic chick. It's simply an annoyance that happens when a blood vessel in the hen's oviduct ruptures while laying. Blood spots occur for all kinds of reasons: vitamin A deficiency, genetics or accidental rupturing. These blobs can happen in fertile eggs as well, but if the egg has never been incubated, it's simply a blood spot, nothing else.
The only way to be 100 percent certain if your egg might be fertile is to crack it open and look inside. Unfertilized eggs will have a small white irregularly shaped layer of protein; this is a blastodisc, a tiny amount of a hen's genetic material. Fertilized eggs contain a blastoderm, which is a perfectly round bull's-eye of pale pre-embryonic cells. Eggs without the bull's-eye are not fertile.
After four days of incubation the blastoderm will exhibit spiderlike veins that spread into the yolk: This is when the embryo begins to develop into a baby chick. After 21 days of incubation, if all goes well, a little fuzzy peeping chick will work his way into the world.
The Difference a Little Warmth Makes
There is a very simple and irrefutable rule about fertilization: An egg that is never incubated will never develop into an embryo. The only difference between an unincubated infertile egg and an unincubated fertile egg is that one has the blastodisc, and the other the blastoderm. Neither of these cell groupings resemble a chick fetus, nor will they ever.
Unincubated fertile and infertile eggs taste the same, look the same, cook the same and include the exact same nutrients: unincubated fertile eggs, remnants of the rooster's sperm are also in the egg.
When incubation occurs, that's when things get interesting. Whether incubated by the mother hen or artificially, fertile egg development begins with consistent warmth, and can be confirmed after Day Four. Fertile eggs laid and refrigerated the same day won't begin to develop at all. An egg must be kept at high temperatures -- around 85 degrees Fahrenheit -- to be considered incubated.
What about Candling?
If you've ever taken a middle school field trip to a farm, you've probably gotten to see some candling done. This procedure shines a bright light -- originally a candle, hence its name -- through an egg's shell to see if a viable fetus is developing. Candling at different stages during the three weeks it takes for an egg to hatch will highlight the chick growing, and its air and yolk sacs diminishing.
Candling before four days of incubation is useless, as there is no embryonic shape to identify. After four days pale veins are faint, but apparent, and after eight days of incubation the dark embryo of the baby chick can be plainly seen. There is little to no point of candling refrigerated eggs, or eggs that have just been laid, as nothing will be revealed.
A chick embryo is not harmed by candling, as long as they are not removed from incubation for an overly long period of time, or the candling device is not too hot. LED lights work best for candling, as they do not emit heat, but do shine brightly.