EGG, the female reproductive cell or ovum of animals, which gives rise generally only after fertilization to a new individual. The largest eggs are those of birds; this because, to the minute essential portion of the egg, or germ, from which the young bird grows, there is added a large store of food-material—the yolk and white of the egg—destined to nourish the growing embryo, while the whole is enclosed within a hard shell.
The relative sizes of eggs depend on the amount of the food yolk thus enclosed ; while the form and texture of the outer envelope are determined by the nature of the environment to which the egg is exposed. Where the food material is infini tesimal in quantity the egg is either not extruded—the embryo being nourished by the maternal tissues--or passes out of the parental body and gives rise at once to a free-living organism or "larva," as is the case in many types of lowly freshwater and marine animals.
The number of eggs periodically produced by any given individ ual depends on the risks of destruction to which they, and the young to which they give rise, are exposed; not more than a single egg being annually laid by some species, while with others the number may amount to millions.
Birds' Eggs.—The egg of the bird affords the readiest example of the modifications imposed by the external environment. Since it must be incubated by the warmth of the parent's body, the outer envelope is a hard shell to protect the chick from pressure, while the dyes which commonly colour the surface of this shell may serve to hide it from egg-eating animals.
Carbonate of lime forms the principal constituent of the shell. In section, it will be found to be made up of three crystalline layers, traversed by vertical canals, whereby it is made porous to admit air to the developing chick.
The outermost layer is often a glaze, as in the ostrich, or it may assume the character of a thick, chalky layer as in some cuckoos (Guira, Crotophaga), cormorants, grebes and flamingoes; while in some birds as in the auks, gulls and tinamous, this outer layer is wanting; yet the tinamous have the most highly glazed eggs of all birds, the second layer of the shell developing a burnished surface.
While some birds' eggs have the shell so thin as to be trans lucent, e.g., kingfisher, others display considerable thickness, the maximum being reached in the egg of the extinct Aepyornis.
Though in shape differing little from that of the familiar hen's egg, certain well-marked modifications of form are to he met with. Thus the eggs of the plover are pear-shaped, of the sand grouse cylindrical, of owls and titmice spherical and of grebes biconical.
In coloration birds' eggs present a remarkable range. The pigments to which this is due have been shown, by their absorption spectra (Sorby, Proc. Zool. Soc., 18i5), to be seven in number. While many eggs are colourless or of one uniform tint, the majority present spots or lines, or both, of varying tints, the pigment being deposited as the egg passes down the lower portion of the oviduct. That the egg during this passage turns slowly on its long axis is shown by the fact that the spots and lines have com monly a spiral direction ; though some of the markings are made during periods of rest, as is shown by their sharp outlines, move ment giving a blurred effect. Where the egg is pyriform, the large end proceeds first. Many eggs display, in addition to the strongly marked spots, fainter spots embedded in a deeper layer of the shell; e.g., rails and plovers.
Among some species, as in birds of prey, the intensity of this coloration is said to increase with age up to a certain point, when it as gradually decreases. Frequently, especially where but two eggs are laid, all the dye will be deposited, sometimes on the first, sometimes on the last laid, leaving the other colourless. But although of a number of eggs in a "clutch"—as the full comple ment in a nest is called—no two are exactly alike, they commonly bear a close resemblance. Among certain species, however, which lay several eggs, one of the number differs markedly from the rest, as in the eggs of the house sparrow. In variability the eggs of the guillemot (Uric troile) exceed all others; both in the hue of the ground colour and in the form of the superimposed mark ings, these eggs exhibit a range for which no adequate explanation has yet been given. Individual peculiarities of coloration are com monly reproduced, not only with this species but also in others, year after year.
Eggs deposited on the bare ground are usually protectively coloured. The eggs of the plover tribe afford striking examples.
But the majority of birds deposit their eggs in a more or less elaborately constructed nest, and in such cases the egg, far from being protectively coloured, often displays tints that would appear rather to attract attention; bright blue or blue spotted with black being commonly met with. It may be, however, that coloration of this kind is less conspicuous than is generally supposed, but in any case the safety of the egg depends less on its coloration than on the harmony of the nest.
The size of the egg depends partly on the number produced, on its surroundings and on conditions determining the state of the young at hatching; hence great disparity in the relative sizes of the eggs of different birds. Young birds which emerge blind, naked and helpless are the product of relatively small eggs, while young hatched from relatively large eggs are down-clad and active from birth.
The fact that the eggs must be brooded by the parent is also a controlling factor in so far as number is concerned, for no more can be hatched than can be covered by the sitting bird. Other less understood factors, however, also exercise an influence. Thus the ,ostrich lays 12 to 16, the teal 15, the partridge 12-20, while among many species the number is strictly limited, as in the hornbills and guillemots, which lay a single egg, the apteryx, divers, petrels, and pigeons never lay more than two, while gulls and plovers never exceed four. Tropical species are said to lay fewer eggs than their representatives in temperate regions.
Partly owing to the uniformity of shape, size and texture of the shell, birds' eggs are by no means easy to identify, except in so far as their family resemblances are concerned; that is to say, except in particular cases, they cannot be specifically dis tinguished, and hence they are of little value for classification.
Save among the megapodes, all birds brood their eggs, the period of incubation varying from 13 days in small passerine birds, to eight weeks in the cassowary. Megapodes deposit their eggs in mounds of decaying vegetable matter or in sand in the neigh bourhood of hot springs, and there leave them. Where the nest ling is active from the moment of hatching, the eggs have a relatively longer incubation period than in cases where the nest lings are for long helpless.
The eggs of the lizards are generally soft-shelled; but the geckos and the green lizard lay hard-shelled eggs. Many of the soft-shelled eggs increase in size after extrusion, owing to the stretching of the membranous shell by the growing embryo. Lizards are less prolific than many chelonians, a dozen eggs being the general number, though as many as 3o may be produced at a time, as in the common chameleon.
While as a rule the eggs of lizards are laid in burrows or buried, some are retained within the body of the parent until the young are ready to emerge; or may even hatch within the oviduct. This occurs with some chameleons and some lizards, e.g., the common English lizard. Normally the young leaves the egg immediately after its extrusion, but if this is delayed it escapes while yet in the oviduct.
The majority of snakes lay eggs, but most vipers and aquatic snakes are viviparous. The shell is always soft and parchment like. As a rule the number of eggs produced is not large-20 or 3o being common—but some pythons lay as many as a hundred. Generally, among oviparous snakes the eggs are buried, but some boas jealously guard them, enclosing them within the coils of the body.
Viviparity occurs among the limbless and the tailed Amphibia, the eggs hatching before they leave the oviduct or immediately after extrusion. The number of young so produced is generally not large, but the common salamander (Salamandra maculosa) may produce as many as 5o at a birth, though 15 is the more normal figure. When the higher number is reached the young are relatively small and weak.
As a rule the young leave the egg as larvae ("tadpoles") ; but many species produce eggs containing sufficient food material to enable the whole larval phase to be completed before hatching.
Among the tailless Amphibia (frogs and toads) there are wide differences in the number of eggs produced, while the methods by which these eggs are disposed of present a marvellous variety. As a rule vast quantities of eggs are shed by the female into the water in the form of "spawn." In the common toad as many as 7,00o eggs may be extruded at a time. These leave the body in two long strings—one from each oviduct—of translucent globules, gelatinous in texture, and enclosing a central sphere of yolk, the upper pole of which is black. The spawn of the common frog differs from that of the toad in that the eggs adhere to form a jelly-like mass. But in many species the number of eggs produced is few; and these may be sufficiently stored with food-yolk to allow of the tadpole stage being passed before hatching, as in frogs of the genus Hylodes. In many cases the eggs are deposited out of the water and often in remarkable ways. In Phyllomedusa the edges of two leaves of a willow overhanging the water are fastened together and the eggs poured into the bag thus formed. The larvae emerging drop into the water. In the midwife toad (Alytes) the eggs are carried about on the hind legs of the male. In the pouched frog, the eggs are carried in a pouch on the back of the female; and in the Surinam toad (Pips) the eggs become embedded in the back of the female, the larval stage being passed within the egg.
In many fishes the eggs float at the surface of the sea, often in enormous masses, when they are carried about at the mercy of tides and currents. An idea of the size which such masses attain may be gathered from the fact that the spawn of the angler-fish, Lophius piscatorius is a sheet 2 to aft. wide, and Soft. long. Another remarkable feature of these floating eggs is their transparency, and hence they probably escape spawn-eating ani mals. The cod tribe and flat-fishes lay floating eggs of this de scription.
The maximum number of eggs laid by fishes varies greatly, but in all cases the number increases with the weight and age of the fish. Thus it has been calculated that the number laid by the salmon is roughly about i,000 to every pound weight of the fish. The sturgeon lays about 7,000,000; the herring 5o,000; the turbot 14,311,000; the sole 134,00o; the perch 280,000. Briefly, the number is greatest where the risks of destruction are greatest.
The eggs of the lampreys and hag-fishes—creatures more primi tive than the true fishes—are remarkable ; in the latter they are large, cylindrical, and provided at each end with hooklets whereby they adhere one to another; while in the lampreys they are small and embedded in jelly.
In variety, in this connection, the Mollusca must perhaps be given the first place. This diversity is illustrated by the eggs of the Cephalopoda. In the squids (Loligo), the eggs are enclosed in long cylindrical cases, of which there are several hundreds, attached by one end to, a common centre ; the whole series look ing like a rough mop-head. Each case, in such a cluster, contains aboit 25o eggs, or about 40,0oo in all. By way of contrast the eggs of the cuttlefish (Sepia) are deposited separately, each en closed in a tough, black, pear-shaped capsule which is fastened by a stalk to fronds of sea-weed or other object. They appear to be extruded at short intervals, till the full complement is laid, the whole forming a cluster looking like a bunch of grapes. The octopus differs again, its eggs being small, berry-like, and attached to a stalk which runs through the centre of the mass.
The eggs of the univalve Mollusca are hardly less varied. In the common British Purpura lapillus they resemble delicate pink grains of rice set on stalks; in Busycon they are disc-shaped, and attached to a band nearly aft. long. The eggs of the shell-bearing slugs (Testacella) are large, and have the outer coat so elastic that if dropped on a stone floor, they will rebound several inches; while some of the snails (Bulimus) lay eggs having a white cal careous and slightly iridescent shell, in size and shape resembling the egg of the pigeon. The beautiful marine violet-snail (Ian thina) carries its eggs on the under side of a gelatinous raft. The eggs of the whelk, like those of the squids, are enveloped in capsules, and these to the number of many hundreds form the large, ball-like masses commonly met with on the seashore.
With the Mollusca, as with other groups where the eggs are exposed to great risks, they are small, produced in great numbers, and give rise to larvae. Thus the common oyster annually dis perses about 6o,000,000 eggs. But where the risk of destruction is slight, the eggs are large and produce young differing from the parent only in size, as in the pigeon-like eggs of Bulimus.
the higher Crustacea, as a rule, the eggs are carried by the female, attached to special appendages on the under side of the body. But in some—Squillas—they are de posited in burrows. Generally they are small so that the young which emerge therefrom differ markedly in appearance from the parents, but in deep-sea and freshwater species the eggs are large, when the young, on emerging, differ little from the adults in appearance.
The eggs of butterflies and moths present a surprising range of variety in shape, sculpturing and coloration. As to the rest of the Invertebrata—above the Protozoa the eggs are laid in water, or in damp places. In the former case they are as a rule small, and give rise to larvae; while eggs hatched on land are sometimes enclosed in capsules, "cocoons," as in the earthworm, in which case this capsule is filled with a milky nutritious fluid on which the embryos feed.
Among some invertebrates two different kinds of eggs are laid. The water-flea, Daphnia (a crustacean), lays two kinds known as summer and winter eggs. The summer eggs are carried by the female in a brood-pouch on the back. The winter eggs, produced at the approach of winter, differ markedly in appearance from the summer eggs, being larger, darker in colour, thicker shelled, and enclosed in a capsule formed from the carapace of the parent's body. \\Tinter eggs, however, may be produced in the height of summer. While the summer eggs are unfertilized, the winter eggs are fertilized by the male, and may lie dormant for months or even years before they develop. The production of these two kinds of eggs is a device to overcome the cold of winter, or the drying up of the pools in which the species lives, during the heat of the summer. The power of resistance such eggs possess may be seen in the fact that a sample of mud which had been kept dry for ten years still contained living eggs. In deep water where neither drought nor winter cold can seriously affect Daphnia, it propagates all the year round by unfertilized summer eggs.