But the embryo enlarging, as we say below, and the ramifications of the meatus venosi extending far and wide to the albumen and vitellus, portions of both of these fluids become liquefied, not indeed in the way Aldrovandus will have it, from some vis insita in the vessels, but from the heat of the blood which they contain. For into whatsoever part of either fluid the vessels in question extend, straightway liquefaction appears in their vicinity; and it is on this account that the yelk about this epoch appears double: its superior portion, which is in juxtaposition with the blunt end of the egg, has already become more diffluent than the rest, and appears like melted yellow wax in contrast with the other colder firmer portion; like bodies in general in a state of fusion, it also occupies a larger space. Now this superior portion, liquefied by the genial heat, is separated from the other liquids of the egg, but particularly the albumen, by a tunica propria of extreme tenuity. It therefore happens that if this most delicate, fragile, and invisible membrane be torn, immediately there ensues an admixture and confusion of the albumen and vitellus, by which everything is obscured. And such an accident is a frequent cause of failure in the reproductive power, (for the different fluids in question are possessed of opposite natures,) according to Aristotle,[183] in the place already so frequently referred to: “Eggs are spoiled and become addled in warm weather especially, and with good reason; for as wine grows sour in hot weather, the lees becoming diffused through it, (which is the cause of its spoiling,) so do eggs perish when the yelk spoils, for the lees and the yelk are the more earthy portion in each. Wherefore wine is destroyed by an admixture with its dregs, and an egg by the diffusion of its yelk.”[184] And here, too, we may not improperly refer to that passage[185] where he says: “When it thunders, the eggs that are under incubation are spoiled;” for it must be a likely matter that a membrane so delicate should give way amidst a conflict of the elements. And perhaps it is because thunder is frequent about the dog days that eggs which are rotten have been called _cynosura_; so that Columella rightly informs us that “the summer solstice, in the opinion of many, is not a good season for breeding chickens.” This at all events is certain, that eggs are very readily shaken and injured when the fowls are disturbed during incubation, at which time the fluids are liquefied and expanded, and their containing membranes are distended and extremely tender. EXERCISE THE EIGHTEENTH. _The fourth inspection of the egg._