100 times greater than a human’s. Each egg is
about 3% of the hen’s weight, so in a year of
laying, she converts about eight times her
body weight into eggs. A quarter of her daily
energy expenditure goes toward egg-making;
a duck puts in half.
The chicken egg begins with the pinhead-
sized white disc that we see riding atop the
yellow yolk. This is the business end of the
egg, the living germ cell that contains the
hen’s chromosomes. A hen is born with
several thousand microscopic germ cells in
her single ovary.
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, p. 74.) When the hen reaches laying
age at between four and six months, the egg