The egg has become as developed as Mrs. Bird alone can make it. For, like the flower seed, the fish and frog egg, it needs the fertilizing substance from the father bird to complete its development. As has been said before, the father bird knows that there are so few eggs that he and Mrs. Bird cannot afford to lose even one, so _great, very great_ care must be taken to fertilize every egg. There is an instinct in all creatures implanted there for millions of years to preserve or perpetuate their species, and this instinct shows itself when the father creature, like the father bird, places himself in such a position that the fertilizing fluid can get into the mother’s body as near as possible to the undeveloped eggs. And as every atom of this substance is alive, it moves on, on and on, until it reaches the egg, where it mingles with it and the two different substances have become one. Now after the two substances have mingled, the egg passes down the little tube on through the opening out of the body into the nest. While it is passing through the tube, however, it accumulates a food substance called the “white” of the egg. This is not the _living_ part of the egg, but simply food just as the frog eggs were incased in a soft substance, which served as food for the tadpole—so in the yolk, is the new baby bird, incased within the “white” of the egg. At the bottom of the tube, through which the egg passes to its opening, is a fluid which also incases the whole egg and hardens into a shell, and it is then ready to go into the little nest. After the fertilization takes place the egg is soon ready to be laid, and so the nest must be ready, soft and cozy. And this is what Mrs. Thrush was now waiting for in the nest she and Mr. Thrush had built in the horse chestnut tree. But even this was not all. For, though the eggs were here, the new little birds were not yet here. And again it was shown that while the frogs and toads left the eggs in the water to care for themselves, Mr. and Mrs. Bird do not and can not do this. The eggs must be kept warm almost constantly, and so Mrs. Bird gathers them close to her warm body and sits on them day after day, until they are ready to burst the shell open, and come out—the real new birds. CHAPTER V. _The Birds and Their Families._ Now that the nest in the horse chestnut tree was completed, Mr. Thrush sat near by waiting to hear the glad news that one egg had come. So the next time Mrs. Thrush went away from the nest for her bath the children peeped into the nest and there saw one speckled egg! The next day another was there, and the next day still another.