7. But if the change from yellow to white has already taken place, neither the barren nor the fertile eggs are altered, so as to change to the nature of the second male. And if the sexual intercourse should be discontinued while the eggs are small, those which existed previously undergo no change, but if the act is repeated, a rapid increase in size takes place. The nature of the white and yolk of the egg is different, not only in colour, but in other properties, for the yolk coagulates with cold, while the white remains fluid, but the white coagulates with heat, which the yolk does not, but remains soft, if it is not burnt; and it becomes consistent and dry by boiling rather than roasting. 8. The white and yolk are separated from each other by a membrane. The chalazæ at the extremities of the yolk have nothing to do with generation, as some persons suppose. These spots are two, one below and one above. If many whites and yolks of eggs are taken out, and mixed together in a vessel, and cooked with a slow and moderate heat, the yolks will all collect in the middle, and the whites will surround them. Young domestic fowls begin to lay eggs at the beginning of the spring; they lay more than those which are older, but those of the young birds are smaller, and if birds are not permitted to incubate, they are destroyed and become sick. 9. After copulation birds ruffle and shake themselves, and often cover themselves with chaff, and this also they do when they have laid. Pigeons draw up their tail, geese go and bathe. The pregnancy and conception of barren eggs is quick in most birds, as in the partridge, on account of the violence of their sexual desires; for if the hen stands in the way of the breath of the male, she conceives, and immediately becomes of no use for fowling; for the partridge appears to have a very distinct smell. The production of the egg after copulation, and the production of the young by incubation, do not occupy the same length of time in all birds, but varies according to their size. The egg of the domestic fowl is perfected in ten days after sexual intercourse, and that of the pigeon in a shorter time. Pigeons are able to retain their eggs even in the act of parturition. If they are disturbed by anything occurring in the neighbourhood of their nest, or a feather be plucked out, or if anything else troubles or disturbs them, they retain the egg they were about to lay.