The formation of an egg occurs in the ovary and oviduct. Although two sets of ovaries and oviducts are present during embryonic development, only the left set fully develop in chickens. The mature ovary will have several follicles in different development stages at any one time (Figure 2). The largest follicle is the one to be ovulated to produce an egg. The ovary will normally produce one mature yolk on a 24h light/dark cycle. The follicular membrane ruptures
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and releases the follicle at the time of ovulation. This usually occurs about 30 minutes after the hen has laid an egg.