Eggs of eggs of birds are so much more in evidence than those of other liv ing creatures that it seems the natural procedure to discuss them as first in order.
The eggs of birds are larger in proportion to the size of the mature parent than is the case with any other division of the animal kingdom. This peculiarity has been "explained" by saying that the bird "must be hatched at an advanced stage of development and therefore much food for it while it is growing "must be stored within the shell of the egg, and that for this . the comparatively large space is required. The matter is mentioned here only to point out that the statement of these facts does not constitute a and also to remark that no reason is necessary it is the normal fashion of develop ment for the bird; that is all there is to it. The egg of a bird originates in the ovary of the female parent. At the very beginning of its course toward the oviduct it is surrounded with a larger or smaller envelope of yolk. Chemi cally this is an emulsion containing varying percentages of fat, lecithin, lutein (the coloring matter), cholesterol, the phosphoprotein vitellin and a small quantity of the coagulable protein livetin. The vitellin and lecithin are in some form of combination and are generally joined (for discussion) under the term lecitho-vitellin. This yolk lies in concentric layers about the whitish germinal vesicle or "tread° of the egg, which is the protoplasmic part where alone de velopment proceeds. This vesicle floats on the surface of the food-yolk and remains on top by reason of its lightness no matter how the egg is turned. The eggs of many of the lower ani mals contain food-yolk, but it is usually color less. The egg of a hen or other bird is further furnished with layers of albumin (the "white") placed upon it in the narrow upper part of the oviduct. Chemically the white of the egg is a coagulable protein fluid of a slightly yellowish tint enmeshed within a network of fibrinous matter. It is feebly alkaline and is 85 to 86 per cent water. Of the solid matter 12.2 per cent is protein — mostly albumin, with smaller propor tions of egg-globulin and ovo-mucoid,— glu cose 0.5 per cent, inorganic salts 0.66 per cent and traces of fat, lecithin and cholesterol. Out side of this there is next deposited upon the albumin a fibrous membrane in two distinct layers (the "egg-podp), the separation of which later forms the "air-Chamber" at the end of the egg where the head of the chick lies. The stages of advancement to this point occupy a period of about four hours. In a further stage of the descent along the oviduct there is deposited in and upon the egg-pod an external covering or "shell calcareous in various degrees. The process of depositing the shell occupies 18 to 20 hours. This shell is porous so as to admit air to the embryo, which also exposes it to bacteria so that eggs are liable to quick decay when living influences cease to operate within it.
Eggs of birds differ greatly in size, color and markings, and this is true even as to differ ent birds of the same species. Although the yolk of the egg is spherical the outer shell is rarely so. It ranges from nearly elliptical or
ovoid to a shape almost conical, known as conoid. It is to be noted that eggs which are laid in exposed places, on rocky ledges or on the bare ground, are much more sharply pointed than those which are laid in deep nests or in holes in trees, and this goes to bear out the theory that the shape of eggs is protective to prevent their rolling away from the nest. In color as a rule they are white, but many are tinted with cream, pink, blue, green or brown, and they may be plain or spotted, speckled or blotched. These blotches and markings are of varying colors and sometimes of several colors on the same egg. They are deposited in the shell of the egg by certain pigment ducts active as it is making; and while the eggs of the whole species have a close general resemblance, so that an ornithologist recognizes them at once, the eggs of individual birds of the species have an individuality of markings. There is often to be observed a double spotting, as if a second layer of shell with another system of spots had been deposited outside of a first layer. It has been suggested that the theory of protective coloring will not fully explain these markings, and that the individuality of their patterns should be ascribed rather to the fact that it affords the mother-bird opportunity for in stant recognition of her own nest. The widest variation in this respect has been noted as to the guillemot, 30 distinctly different patterns of markings being shown in some collections. It has been observed also that the markings on the eggs of birds in captivity vary with the state of health and as to conditions of mind, as fear, anger and the like. It is notable that eggs which are laid in dark places are as a rule with out markings. It is to be expected as a matter of course that eggs would differ in size just as do the parent birds. A novice in the study of bird lore, however, is not prepared for the wide disparity in relative size when the eggs of dif ferent species are compared. The smallest egg known among birds is that of the humming bird, the smallest of the birds. This is quite as one would expect. But the cuckoo lays an egg which is but one-fifth of the size of that laid by the jackdaw, although the two parent birds are of the same size. The mature guille mot is about the same size as the raven, but its egg is three times larger. The egg of the New Zealand wingless bird, the kiwi, has a pro digious weight, fully one-fourth that of the parent, while the egg of the ordinary barnyard hen, which may be taken as typical, weighs about one-thirtieth as much as the adult fowl. The largest known bird's egg is that of the now extinct epyornis of Madagascar, which was 10 inches in its smallest diameter. The largest egg of species now in existence is that of the ostrich. Variations in the size of eggs, moreover, may occur in the same species in a different habitat. Thus the eggs of the great white heron in Europe arc 40 per cent larger than those of the same species in India.