EGG, in Physiology, a body formed in certain females, in which is contained an embryo or foetus of the same species, under a cortical surface or shell. The exterior part of an egg is a shell, which in a hen, for instance, is a white, thin, and friable cortex, including all the other parts. The shell becomes more brittle by being exposed to a dry heat. It is lined everywhere with a very thin but pretty tough membrane, which dividing at or very near the obtuse end of the egg, forms a small bag, where only air is contained. In new-laid eggs this follicle appears very little, but becomes larger when the egg is kept. Within this are contained the albumen or white, and the vitellus or yolk; each of which has its different virtues. The albumen is a cold, viscous, white liquor in the egg, different in consistence in its different parts. It is observed, that there are two distinct albumens, each of which is inclosed in its proper membrane. Of these, one is very thin and liquid, the other is more dense and viscous, and of a somewhat whiter colour, but, in old and stale eggs, after some days incubation, inclining to a yellow. As this second albumen covers the yolk on all sides, so it is itself surrounded by the other external liquid. The albumen of a fecundated egg is as sweet and free from corruption during the whole time of incubation as it is in new laid eggs; and so is also the vitellus. As the eggs of hens consist of two liquors separated one from another, and distinguished by two branches of umbilical veins, one of which goes to the vitellus, and the other to the albumen; so it is very probable that they are of different natures, and consequently appointed for different purposes. When the vitellus grows warm with incubation, it becomes more humid, resembling melting wax or fat; and hence it occupies more space. For as the foetus increases, the albumen insensibly wastes away and condenses; the vitellus, on the contrary, seems to lose little or nothing of its bulk when the foetus is perfected, and only appears more liquid and humid when the abdomen of the foetus begins to be formed. The chick in the egg is first nourished by the albumen, and, when this is consumed, by the vitellus, as with milk. If we compare the chalaza to the extremities of an axis passing through the vitellus, which is of a spherical form, this sphere will be composed of two unequal portions, its axis not passing through its centre; consequently, since it is heavier than the white, its smaller portion must always be uppermost in all positions of the egg. The yellowish white round spot, called cicatricula, is placed on the middle of the smaller portion of the yolk; and therefore, from what has been said above, must always appear on the superior part of the vitellus. Not long before the exclusion of the chick, the whole yolk is taken into its abdomen; and the shell, at the obtuse end of the egg, frequently appears cracked some time before the exclusion of the chick. The chick is sometimes observed to perforate the shell with its beak. After exclusion, the yolk is gradually wasted, being conveyed into the small guts by a small duct.

Eggs differ very much, according to the birds which lay them, as to their colour, form, size, age, and the different ways of dressing them: those most used in food are hens’ eggs. As to the preservation of eggs, it is observed that the egg is always quite full when it is first laid by the hen; but from that time it gradually becomes less and less so, till its decay; and however compact and close its shell may appear, it is nevertheless perforated with a multitude of small holes, though too minute for the discernment of our eyes, the effect of which is a daily decrease of matter within the egg, from the time of its being laid. This perspiration is much quicker in hot w cather than in cold. To preserve the egg fresh, there needs no more than to preserve it full, and stop its transpiration; the method of doing which is, by stopping up those pores with matter which is not soluble in watery fiuids; and on this principle it is that all kinds of varnish, prepared with spirit of wine, will preserve eggs fresh for a long time, if it be carefully rubbed all over the shell. Tallow, or mutton fat, is also good for this purpose.

Artificial Method of Hatching Eggs. See Hatching.