My mother gave me a fabulous old book that you would enjoy – Window Into An Egg: Seeing Life Begin (1969) by Geraldine Lux Flanagan, especially if you are homeschooling or incubating in your classroom. You can find used copies of this book on Amazon, Ebay, or perhaps your local library. Unfortunately, it is no longer in print. Photographs from microscopes and within eggs are combined with fantastically detailed and accurate descriptions of the scientific processes involved in the development of a chick. Unlike most books for kids, this information is not dumbed-down or over-simplified; it is clearly explained.
But before we look into our egg to see how a chick is made, let us see how the egg itself is made and then is laid by the hen. As you remember, when the ovum is fertilized, it has no shell around it. Only the yolk lies there, deep inside the hen’s body, in a special egg tube. The yolk is enclosed by a very fine transparent envelope that holds the ball of yolk together and also covers the snowflake patch on it. The envelope is so thin that the rooster’s sperm can get through it and can reach the white patch, which is called the BLASTODISC, in Greek meaning “growing disc.” The yolk and the white patch and the envelope around them, these three are the true egg, or ovum. The rooster’s sperm must reach this ovum inside the hen while the ovum is still bare, before the egg white and the eggshell have covered it.
Egg white and eggshell are extra food and housing for the chick…
– Flanagan, p. 19-20
UPDATE: See http://chickscope.beckman.uiuc.edu/explore/embryology/ for very similar pictures and great explanations!