This hollow place on the end of the boiled egg (sometimes it shows on the side) is the air which is put inside of the egg when it is formed so that the little chicken will have air to breathe from the time it comes to life within the egg until it becomes strong enough to break the shell and go out into the world. There is also food in the egg for him. When you boil the egg this pocket of air within the shell, which would have been used up by the chick if the egg had been set to hatch instead of being cooked for breakfast, begins to fight for its space and pushes the boiling egg back and forms the hollow place. The purpose of the air in the egg is a good thing to remember when we come to study the higher forms of animal life from the standpoint of how they reproduce themselves. The mammals are the next higher form of animals. The babies of this class of animals must be fed for several weeks or months before they are ready to come into the world. A little chicken is ready to come out of the egg almost as soon as it comes to life, and, therefore, needs only a little air and food before it is strong enough to peck its way out, but the babies of mammals begin to live months before they are ready to come into the world, and they need a great deal of air and food during this time. This class includes the dogs, horses, cows, cats and all other animals in the Zoo and in the woods. The name mammals means the same as “mamma,” and indicates an animal which must be fed from the body of a female mammal even after it is born. In this class the eggs are retained within the body of the female animal instead of being laid in a nest or some other place, as in animals of lower classes, after being fertilized by the male animal, so that the baby animal may secure its food and air from within the mother’s body after the life within the egg is begun. The mother’s body supplies the necessary warmth to develop the life of the little animal in the egg, just as the birds supplied this with their bodies. In the bird class it only takes a few hours to give the little bird sufficient strength to peek his way out, but in the mammal class it is a long time before the baby animal is strong enough to come out into the world, and even after it is born the babies of mammals require a great deal of care and attention before they are able to look out for themselves. During this period the animal secures all of its food from the breast of the mother animal.