The starch and the oil of seeds, the sugar of fruit, and the lesser quantities of nitrogen contained in all seeds, are in a more available form for cell-nourishment than would be the original mature portions of plant life.
Milk and eggs in the animal world occupy a position identical to that of seeds and fruit in the plant world; that is, they are created for the first nourishment of the offspring.
In the process of evolution, a fundamental distinction between birds and mammals is in the manner in which the young are nourished. The egg of the bird supplies sufficient nourishment to develop the young bird to a point where it can exist upon the ordinary food of the adult bird.
The hen's egg must contain all food material necessary to form all portions of the body of the chick, and to supply it for a time with heat and energy.
An average egg weighs two ounces; of this weight about 10 per cent is shell, 30 per cent yolk, and the remainder white. The white of the egg is composed of albumin and water. The yolk consists of globulin, egg-fat, and lecithin; this latter substance contains a considerable proportion of phosphorus, and is one of the essential contingents of brain and nerves. The egg-shell contains 13 per cent protein, 10 per cent fat, and one per cent ash.
[Sidenote: Milk and eggs not a balanced adult diet]
The younger the animal, the more rapid is the growth of the animal body compared with the amount of energy expended. For this reason the percentage of nitrogen in milk and in eggs is much too great to form a balanced adult diet, and should be supplemented by articles containing larger proportions of heat-producing materials, preferably carbohydrates.
[Sidenote: Eggs for emaciation and convalescents]
The proteid material of eggs is in a form especially adapted to the construction of new cells. For this reason it is one of the best known foods for use in cases of emaciation, where new tissue is to be added rapidly to the body. An egg contains about fourteen decigrams of nitrogen. Ten eggs, therefore, would supply an ample amount of nitrogen for the daily needs of the average body, were no nitrogen taken from other sources. In feeding patients who are convalescing from fevers or other wasting dis-eases, it is sometimes necessary to prescribe a diet of from ten to twelve eggs daily for a limited time.