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The ovaries of the bird are in the small of the back close to the backbone, and there is a tube called the oviduct or egg-duct, leading from the ovary down to the lower end of the intestine, which it enters. There is no separate opening for the oviduct into the outer world.
The yolk in short is the egg. But there is not enough food material in it for the development of the bird, so as it passes down the egg-duct it becomes coated by the so-called "white" of the egg, which is a substance secreted from the lining of the egg-duct and is not alive, as is a certain part of the yolk.
It is merely stored-up food like that in the morning-glory seed, for this egg is the seed of the bird. At the lower end of the egg-duct there is secreted a limy liquid which covers the shell-less egg and hardens, making the shell. So finally the fertilized egg has its shell and is ready to be laid.
When the mate has been chosen, both prepare the nest to receive the eggs, which will soon be ready. It is during this period that the fertilizing fluid is placed in the lower end of the egg-duct, whence the fertilizing cells, by their power of motion, quickly make their way to the egg, which has just begun its journey down the oviduct and is as yet without a shell.
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