The infant was first an embryo. Its body is neither new nor untried. Its heart, its liver, lungs and brain, all its nerves and tissues, were in existence some time before, constantly exercised in embryonic ways. The new born infant is an old campaigner on quite familiar terrain.

The embryo begins as a single cell feeding on the food its mother provides. Almost at once it becomes too large, too sluggish and inefficient for its own good, and must either immediately change its tactics and mode of life or be prepared to die. This is an alternative which all growing things face from time to time. The lowest organisms meet the issue by subdividing into a number of similar, smaller, independent cells, each with a life of its own. The embryo meets it by subdividing into a number of different kinds of cells all of which are capable of developing and living according to individual rhythms, but only a few of which are able to exist or develop all alone. Most of the new cells the embryo makes possible must cling to and remain under its sway if they are to continue and prosper at all.