This image depicts the appearance of a healthy, 11-day-old fertilized egg when observed using what is known as a 'candling' procedure. A very bright light is either placed behind the egg, such as was done throughout history by using a candle, hence the name, or employing more modern methods, using a powerful lamp placed against the broad end of the egg. In this way, the contents of the egg are revealed through the translucent shell. If the egg is unfertilized, as was the case seen in BM8171, the observer will see only a round yolk sac, and no developing embryo, or any network of blood vessels. Unfertilized eggs are known as 'yolkers'. Eggs known as 'quitters', had been fertilized, but the embryo had stopped growing. In the case of quitters, one will note a thin blood ring encircling the yolk (see BM8157). As was the case with this egg, 'winners' are eggs that had been fertilized, and lead to the growth of a healthy embryo. In this case, one will observe numbers of networked blood vessels surrounding the yolk, and depending upon the length of gestation, one might see a dark shadow representing the developing embryonic eye. Candling is used in the production of influenza vaccines. It allows unfertilized or defective eggs to be identified, and discarded.