Among animals which produce eggs, the yolk is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example because they are laid in situations where the food supply is sufficient or because the embryo develops in the parent's body, which supplies the food, usually through a placenta. Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophic; those in which the embryo is supplied by yolk are said to be lecithotrophic. In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fish and invertebrates, the yolk material is not in a special organ, but inside the egg cell.

सबसे पहले हम आपको बताना चाहेंगे कि अंडे के पीले भाग को जर्दी के नाम से भी जाना जाता है। वहीं, अंग्रेजी भाषा में इसे Egg Yolk भी कहते हैं। आगे चलकर यही भ्रूण की शक्ल लेता है जिसके कारण एक चूजा तैयार होता है