Making the Yolk

As the hen grows, her germ cells gradually reach a few millimeters in diameter, and after two or three months accumulate a white, primordial form of yolk inside their thin surrounding membrane. (The white yolk can be seen in a hard-cooked egg; see box.) When the hen reaches laying age at between four and six months, the egg cells begin to mature, with different cells at different stages at any given time. Full maturation takes about ten weeks. During the tenth, the germ cell rapidly accumulates yellow yolk, mostly fats and proteins, which is synthesized in the hen’s liver. Its color depends on the pigments in the hen’s feed; a diet rich in corn or alfalfa makes a deeper yellow. If the hen feeds only once or twice a day, her yolk will show distinct layers of dark and light. In the end, the yolk comes to dwarf the germ cell, containing as it must the provisions for 21 days during which the chick will develop on its own.