We have to do, then, with the two fluids of the egg, the albumen and the vitellus ; for these, before all the other parts, are formed for the use of the embryo, and in them is the second action of the egg especially conspicuous.
The egg of the common hen is of two colours internally, and consists of two fluids, severally distinct, separated by mem- branes, and in all probability of different natures, and there- fore having different ends to serve, inasmuch as they are distinguished by different extensions of the umbilical veins, one of them proceeding to the white, another to the yelk. " The yelk and white of the egg are of opposite natures," says Aris- totle, 1 " not only in colour, but also in power. For the yelk is congealed by cold ; the white is not congealed, but is rather liquefied ; on the contrary, the white is coagulated by heat, the yelk is not coagulated, but remains soft, unless it be overdone, and is more condensed and dried by boiling than by roasting." The vitellus getting heated during incubation, is rendered more moist ; for it becomes like melted wax or tallow, whereby it also takes up more room. For as the embryo grows, the albumen is gradually taken up and becomes inspissated ; but the yelk, even when the foetus has attained perfection, appears scarcely to have diminished in size ; it is only more diffluent and moist, even when the foetus begins to have its abdomen closed in.
Aristotle 2 gives the following reason for the diversity : " Since the bird cannot perfect her offspring within herself, she produces it along with the aliment needful to its growth in the egg. Viviparous animals again prepare the food (milk) in another part of their body, namely, the breasts. Now nature has done the same thing in the egg ; but otherwise than as is generally presumed, and as Alcmseon Crotoniates states it, for it is not the albumen but the vitellus which is the milk of the egg."
For as the foetus of a viviparous animal draws its nourish- ment from the uterus whilst it is connected with its mother, like a plant by its roots from the earth ; but after birth, and when it has escaped from the womb, sucks milk from the breast, and thereby continues to wax in size and strength, the
1 Hist. Anim. lib. vii, cap. 2. 2 De Gen. Anim. lib. iii, cap. 2.
