“The vitellus,” says Fabricius,[152] “is so called from the word vita, because the chick lives upon it; from its colour it is also spoken of as the yellow of the egg, having been called by the Greeks generally, χρυσὸν, by Hippocrates χλωρὸν, and by Aristotle ώχρὸν and λεκυθὸν; the ancients, such as Suidas in Menander, called it νεοττὸν, i. e. the chick, because they believed the chick to be engendered from this part. It is the smoothest portion of the egg, and is contained within a most delicate membrane, immediately escaping if this be torn, and losing all figure; it is sustained in the middle of the egg; and in one egg is of a yellow colour, in another of a tint between white and yellow; it is quite round, of variable size, according to the size of the bird that lays the egg, and, according to Aristotle, of a deeper yellow in water birds, of a paler hue in land birds.” The same author[153] also maintains that “the yellow and the white of an egg are of opposite natures, not only in colour but in qualities; for the yellow is inspissated by cold, which the white is not, but is rather rendered more liquid; and the white, on the contrary, is thickened by heat, which the yellow is not, unless it be burned or over-done, and it is more hardened and dried by boiling than by roasting.” As in the macrocosm the earth is placed in the centre, and is surrounded by the water and the air, so is the yelk, the more earthy part of the egg, surrounded by two albuminous layers, one thicker, another thinner. And, indeed, Aristotle[154] says that, “if we put a number of yelks and whites together, and mix them in a pan, and then boil them with a slow and gentle fire, that the whole of the yelks will set into a globular mass in the middle, and appear surrounded by the whites.” But many physicians have been of opinion that the white was the colder portion of the egg. Of these matters, however, more by and by. The chalazæ, the treads or treadles (gralladura Ital.) are two in number in each egg, one in the blunt, another in the sharp end. The larger portion of them is contained in the white; but they are most intimately connected with the yelk, and with its membrane. They are two long-shaped bodies, firmer than the albumen and whiter; knotty, not without a certain transparency like hail, whence their name; each chalaza, in fact, is made up of several hailstones, as it seems, connected by means of albumen. One of them is larger than the other, and this extends from the yelk towards the blunt end of the egg; the other and smaller chalaza stretches from the yelk towards the sharp end of the egg. The larger is made up of two or three knots or seeming hailstones, at a trifling distance from one another, and of successively smaller size.