7. At this period also they eject an excrementitious matter into the external chorion, and contain it in the stomach. The external excrement is white, the internal yellow. At last the yolk, which has been continually wasting and advancing, is entirely taken up and enclosed in the young bird. So that portions of it may be observed in the intestines of birds if they are dissected on the tenth day after exclusion from the egg. But it is set at liberty from the navel, nor does any communication remain, but the whole is separated. About the before-mentioned period the young bird sleeps, but it stirs itself, and looks up, and chirps when it is touched, and the heart swells up with the navel, as if the embryo were breathing. This is the manner of the development of the chick in the egg. 8. Birds also produce some barren eggs, as well as those from sexual intercourse, but they produce nothing after incubation. This is particularly observed in pigeons. Double eggs have two yolks; in some a thin division of white prevents the yolks from mixing together; others have not this division, but touch each other. There are some hens which always lay double eggs, and in these the peculiarities of the yolks have been observed; for a certain bird having laid eighteen eggs, hatched two chickens from each of them, except those that were addled; all the rest were productive, except that one of the twin chickens was large and the other small in each. The last, however, was monstrous. CHAPTER IV. 1. All the pigeon tribe, as the phatta and trygon, generally produce two eggs; the trygon and the phatta are those which generally lay three. The pigeon lays, as I said, at every season; the trygon and the phatta in the spring, and not more than twice. The second brood are hatched when the first has been destroyed, for many birds destroy them. It sometimes lays three, as I have said, but it never brings out more than two young ones, and sometimes only one, the remaining egg is always addled. Very few birds begin to lay before they are a year old; but when they have once begun to lay, they all, as we may say, naturally contain eggs to the end of their life, though it is not easy to see them in some birds, from their small size.