PARTS OF A FOWL'S EGG. Diagrammatic section of the fertilized egg of a hen: s,

shell; sm, shell-membrane or egg-pod, split at the blunt pole of the egg to make the air-chamber (a); w, albumen layers or 'white'; ch, ch, chalaza; y, yellow food-yolk, in concentric layers, around the white yolk, or germinal vesicle (g); bl, germinal disk, blastoderm, or tread.' tureless, either very poor in calcine salts, and in this case smooth and shiny, or considerably infiltrated with calcareous matter, and then exhibiting the well-known chalky and often rough appearance of the eggs of the ani, cormorants, grebes, and flamingoes. Even when well developed this cuticular layer is always extremely thin. In the ostrich and in rhea it is hard and brittle, like the glaze of pottery; in the common fowl and turkey it is parchment-like; in auks and apparently in gulls it is absent. The cuticle is spread over the whole surface of the egg, and into the pits or surface-ends of the air-canals, which are therefore closed when such a cuticle is present. The latter, however, readily admits the passage of air when dry, but when wet or moist is impermeable to air." The very great differences observable in the surface of eggs, from rough and pitted to enameled or polished, or translucently brilliant, is due to the varying crystalline structure of the shell. This has been very thoroughly worked out by German microscopists, whose extensive investigations are recorded in the Journal für Ornithologie for 1871 and subsequently, and show that a well-defined type of shell-structure belongs to each of various

groups.

The porosity of the shell is needful, of course, in order to admit air to the embryo; and the pores are large enough to admit also some bacteria, so that after a short time eggs not energized by vital processes become corrupt and rotten. The various preservative processes that have been attempted to prevent decay depend for their success upon the complete exclusion of bacteria, and only succeed when they are begun before any infection has been received, and when they absolutely close all the pores of the shell. This can best be done by soaking in lime-water, in effect reinforcing and completing the natural

shell.

NUMBER AND SIZE OF EGGS. The number of eggs that constitute a 'clutch' or 'set' is fairly uniform with each species, but differs greatly among different species. Pelagic water-fowl, and the larger birds of prey usually lay only one egg, rarely more than two; many game birds and lake and river fowl lay a dozen; wood

land and field birds average five, though in some species two or three are prevailing numbers, and in others, for example titmice, the clutch may consist of ten eggs. The number of eggs laid, except in cases correlated with peculiar circumstances in life history are SO nearly constant among species of similar habit and bear so evident a relation to the average comparative danger to which the species is exposed, that naturalists recognize evidences of a seeming general principle which governs the size of the brood, and if any peculiarity should appear in the nesting or the rearing habits that might accrue to the welfare of the individual species, it is offset by a reduced reproductive power.

Another noteworthy fact is that great diversity exists in the size and weight of eggs as compared with the size and weight of the mother bird. Hewitson states, for example, that the raven and guillemot are of about equal size, but their eggs vary as four to one, the latter's being as big as those of an eagle. Still more remarkable for disproportionate bigness are the eggs of the Australian mound-birds, which measure 34 by 2% inches, though the hen is only about the size of a common fowl; and of the extinct moas, epiornis and the existing kiwis (Apteryx), that of one of the kiwis weighing nearly one-fourth as much as the hen which produces it. Much individual variation exists, and several factors seem to enter into the explanation; but in general eggs much larger than ordinary, relatively to the size of the mother, belong to birds whose precocial young will receive little or no help and care from their parents after birth, and hence must remain within the protection of the egg until they are considerably advanced toward maturity. This requires a very much larger egg than is needed for the early-born, parent-supported, altricial (yolk) must be stored up for them, and they birds, since a much greater quantity of food must have room to grow. It is plain that few the mother's organization, and as a matter of such eggs can be produced from the resources of fact these huge eggs are laid singly; as a rule, relatively large eggs mean few in a brood. SHAPE.

In shape birds' eggs are rarely spherical, though the contained yolk is always so, but ordinarily is 'ovoid,' being much larger at one end than the other, and often nearly conical; while some are oblong, having a decided length but no excess of thickness at either end. Eggs laid in holes or deep, cup-like nests, where they cannot fall out, are likely to be spherical or ovoid; while those laid upon the ground, with little or no nest, are usually conoid, and likely to be large for the size of the bird. This shape gives them two advantages: first it enables them to lie more closely together, the points toward a common centre, so that they are easily covered by the brooding bird; and, second, in the case of eggs, like those of guillemots, which are laid upon bare ledges of sea-cliffs with no encircling nest. the conical shape prevents their being easily rolled or blown away when uncovered, because when moved they will tend to gyrate upon their points and not roll away. Among the services which the egg-shell performs the foremost are the protection of the embryo from injurious changes of temperature, and from too much light. See GROWTH.

COLOR. Birds' eggs may be said to be normally chalky white, but in a great number of groups they are colored. The color may be a uniform tint over the whole surface, or a tint with markings; or simply dots, spots, streaks, or blotches of one or more hues on an otherwise colorless shell. These colors exude from uterine glands in the form of pigment-corpuscles deposited with the forming shell, and stain its deeper as well as its more external layers. Many of the variations in tint of the spots are thus due simply to their being overlaid by more or less transparent material, but certain real colors exist. This matter was studied by H. C. Sorby by the methods of spectrum analysis described by him in the Proceedings of the Zoological Society of London for 1875. He learned that the coloring matters in egg-shells are connected with hæmaglobin and the bile-pigments, and are definite physiological products. He reported seven of these products as follows:

(1) Oörhodeine. The most important of the colors, present in almost all eggs, which gives an effect when alone of brownish red. The spectroscope shows so close a similiarity between this and the product of the decomposition of the red corpuscles of the blood as to make it probable that they are physiologically identical, and derived from the same source. Such red-brown eggs as those of the grouse are mainly tinted by this material.

(2) Oöcyan. A fine blue.

(3) Banded Oöcyan. Also a fine blue, but giving a different spectrum. A chemical similarity is apparent between the two oöcyans and the pigments of the bile, and they must be regarded as derived by separate physiological processes from the same source.

(4) Yellow Oöxanthine. Clear yellow; unstable and liable to fade.

(5) Rufous Oöxanthine. Reddish yellow. (6) A substance giving narrow absorptionbands in the red. Probably brown.

(7) Lichenoxanthine. A brick-red substance, apparently identical with a coloring matter common in plants, especially in lichens and fungi.

The mixture of these pigments in various ways gives the varied colors observed in eggshells. Thus the nightingale's egg has been shown to be colored with a mixture of oörhodeine and oöcyan; while all the varying greens and blues so common as ground-tints are due to varying mixtures of oocyan with oöxanthine, and such are likely to fade or to change color when long exposed to the light.

An interesting feature in this connection is that not only the state of health, but the state of mind of a bird, may effect the colors of its eggs. Hewitson long ago recorded the observation that birds captured on their nests and placed in confinement deposited eggs "deprived of much of their color." Colorless eggs are frequently found in the nests of birds normally laying colored eggs; and great variety and inconstancy in tint and pattern are always observable.

THEORIES OF COLOR-MARKINGS ON EGGS. The reason for the many and various colorings of birds' eggs has been a favorite field of speculation, and many ingenious theories have been constructed to explain the matter, from the pious musings of Sir Thomas Browne to the present day. The most important thesis is that advanced by Wallace and enlarged by Poulton, who attempt to show

that birds' eggs are examples of protective mimicry in color, as the result of natural selection. Impartial examination shows, however, that only in a minority of cases does the theory of adaptive coloration seem to be adequate and not open to serious interference from other explanations or contradictions: such favorable cases are those of the nightjars, game birds, shore birds, coots, and some other ground-builders making scanty nests, whose eggs certainly do closely simulate the beach, or leaves, or marsh-grass upon which they lie. They are certainly very difficult to see, especially for human eyes; but most of the despoilers of birds' nests are not human, but brute foes which, for the most part, depend less upon their eyes than upon their noses to discover what they seek, and most of which go about by night rather than by day. Against such foes color is of small importance among the factors of safety. It is quite probable that in the cases above mentioned the inconspicuous coloring of the eggs is of some consequence, and may be the result in some degree of natural selection; but these cases are only a small part of the whole array of bird life, and 'protective mimicry' certainly fails as yet to account for the coloring of birds' eggs as a whole.

It is much more likely that this phenomenon falls under the theory of 'recognition' colors. Many facts go to show that birds recognize their own eggs, and the supposition is reasonable that the tints and markings, if they serve any useful purpose at all, serve that of identification by their owners. This would account for the fact that eggs laid in dark holes are usually white; and it would explain the individual variation within the limits of specific or tribal likeness which universally characterizes the eggs of birds.

UTILIZATION OF BIRDS' EGGS. Eggs being intended not only for the production of new beings, but for their nutrition during the embryonic period, mainly consist of a store of highly nutritious material, which is as good food for other animals as for the young birds. Hence all eggs may be called edible, though some are so impregnated with a fishy or musky flavor or odor as to be unpalatable to civilized mankind. The vast use made of the eggs of domestic fowls as food and in the arts is elucidated in another article (see EGG AS A FOOD), but attention may be called here to the fact that in many parts of the world the gathering of the eggs of birds, mainly sea-birds, which nest in vast communities, is a regular industry furnishing an important part of the food of the local population. This is especially true of high northern regions. In some of the islands of the Pacific eggs of gre garious sea-fowl are gathered by thousands, for the purpose of making from them commercial albumen; but this industry is diminishing by reason of the steady depletion of the numbers of the sea-fowl thus disturbed and robbed. The making of collections of egg-shells for scientific museums, and still more for the satisfaction of private curiosity and ambition, has long been an industry in which much money is annually expended, rare eggs often commanding very high prices, the few eggs remaining of the extinct great auk (q.v.) being worth at present more than $2000 each. Lastly it is to be mentioned that in the season when birds are breeding their eggs and fledglings form an important part of the food of many other animals.

MAMMALIAN EGGS. The eggs of all the mammals except the monotremes (q.v.) are minute globules (that of human beings being only about of an inch in diameter) containing little yolk, which pass through all their stages of development within the body of the mother. (See EMBRYOLOGY.) The two groups of monotremes, the duckbills and echidnas (qq.v.), however, are ovoviviparous, that is, they produce eggs which are voided from the mother and developed outside her body. These eggs are very much larger than those of other mammals, because they must carry a comparatively large amount of yolk-food. Those of the duckbill, two in number annually, are globular, about threefourths of an inch in diameter, and have a tough white calcareous shell; they are deposited in a soft nest in the animal's burrow and hatch there. The echidna produces one egg a year, one-half to three-quarters of an inch in diameter, and covered with a leathery envelope (keratin), which is placed as soon as voided in a mammary pouch and there completes its development.

or less advanced stage of development each egg is covered by a dark-brown chitinous case, which most commonly is flat and four-cornered, with twisted filamentous appendages at the corners, by means of which it becomes attached to seaweeds and the like. These are the sea-purses' of fishermen.

EGGS OF REPTILES AND AMPHIBIANS. Most reptiles are oviparous, laying eggs, either globular or oblong in shape, closely resembling birds' eggs in composition of yolk, albumen, and coverings, except that the shell is not calcareous, but of a tough, yellowish-white, parchment-like (coriaceous) material. "These are usually deposited in holes and left to hatch by the heat of the sun. In the [case of] the crocodiles they are deposited in a rough nest, and guarded by the mother. In all cases development has only progressed to a very early stage when the deposition of the eggs takes place, and it is only after a more or less prolonged period of incubation that the young, fully formed in every respect, emerge from the shell and shift for themselves." These eggs are most numerous in the case of the turtles, numbering from 25 to 250, and are buried in the sand of sea-beaches and river-banks. Those of the fresh-water species are mostly nauseous, but the eggs of sea-turtles are excellent food, will keep fresh a long time, and are extensively gathered, the people of many tropical countries subsisting largely upon them in their season. They are also much fed upon by wild animals. Many lizards and serpents do not lay eggs, but are viviparous; but the pythons not only lay them, but incubate them within the circle of their coiled bodies.

THE EGGS OF THE AMPHIBIANS. These differ from eggs of reptiles in being small, numerous, having comparatively little food-yolk, and are usually laid in the water inclosed in gelatinous masses or cords. Sometimes they float or lie upon the bottom, or attached to submerged plants, or are laid under logs or stones; and in a few cases they are carried about by one or the other parent until they hatch, or the mother coils about them in an underground cell. They are devoured by some aquatic animals.

The bony fishes mostly emit minute eggs, usually called 'spawn,' in vast quantities, a single sea-fish producing two or three hundred thousand in some cases. These are cast into the water and contribute a large part of the food-supply of aquatic creatures, nineteen-twentieths, probably, being quickly devoured. Some float at or near the surface; others sink to the bottom. Some fresh-water fishes, however, deposit their eggs in prepared nests, where they are guarded and attended until they hatch. (See NIDIFICATION.) This reduces the proportion of loss so greatly that comparatively few need be produced, and they are far larger, relatively, than in the other case. The eggs of such fishes as the shad, under the name of 'roe,' and of the sturgeon (caviar), enter largely into the list of human comestibles.

EGGS OF INSECTS. Eggs and their deposition among the insects present a great variety of interesting phenomena, of which only a sketch may be given here. In many cases in this class the eggs are so carefully placed as to insure the survival of almost every one, and in proportion as the precautions are complete the number periodically laid is diminished; in fact, in nearly all cases, except in the Hymenoptera, each egg, or else a group of eggs, is within a strong shell or capsule.

Bees, wasps, and ants lay relatively large, globular, shell-less eggs, usually each by itself, in the cells of their combs or burrows, placing with them food for the expected larvæ (honey, beebread, palsied spiders, etc.). Other insects place them within or upon the bodies of other insects, as is the habit of the parasitic ichneumons and chalcids; or else insert them into plant-tissues and wood, as do the gall-flies, sawflies, etc. Many flies produce living larvæ, and others retain their eggs until nearly mature. The bottle-shaped eggs of mosquitoes stick together in a sort of cake

or raft that floats on the surface of water; those of the midges are imbedded in jelly, which swells in the water and sticks to a support; the buffalognats glue theirs in large patches to submerged rocks. The snipe-flies, gadflies, and some others put their eggs on dried branches overhanging the water, sometimes in large pear-shaped masses to which several females contribute, and many others force theirs into the ground or into decaying wood or dung, carrion meat, or wounds, as is the case with house-flies, flesh-flies, syrphusflies, etc.; while the bots attach theirs to animal hairs, where they may be licked off and develop in the stomach of their host. It is clearly impossible to carry this particular description through the list. One of the longest as well as most entertaining chapters in the admirable Introduction to Entomology, by Kirby and Spence, is filled with such details. An excellent summary has been made by Carpenter as follows: "The outer form of insects' eggs is exceedingly variable. Very many-as those of beetles, grasshoppers, and flies-are elongate, like the cockroach's. The eggs of some moths are globular, while those of butterflies, and especially those of