Among serpents the viper is externally viviparous, but first of all internally oviparous. The ovum, like that of fish, is of one colour and soft skinned. The young are produced in the upper part. They are not enclosed in a shelly covering, neither are the ova of fish. The little vipers are produced in a membrane, which they rupture on the third day, and sometimes they make their escape by eating their way through the mother. They are produced one by one in the course of a day, and their number often exceeds twenty. Other serpents are externally oviparous, but their ova are joined together like women's necklaces. When the female deposits her eggs in the soil, she incubates upon them. These also are hatched in the second year. This is the manner of the production of serpents, insects, and of oviparous quadrupeds. BOOK THE SIXTH. CHAPTER I. 1. The above describes the manner of reproduction in serpents, insects, and oviparous quadrupeds. All birds are oviparous, but the season of sexual intercourse and of bringing out their young is not the same in all; for some copulate and produce eggs at all seasons, as we may say, as the domestic fowl and the pigeon, for the domestic fowl lays eggs all the year round, except two months at the winter solstice. Some of the finest birds will lay sixty eggs before they want to sit, though these are not so fruitful as the more common kinds. The Adrianic fowls are very small, but they lay every day; but they are cruel, and often kill their chickens. Their colour is variegated. Some of the domestic birds lay twice a-day, and some have been known to lay so many eggs that they died very soon. 2. The domestic fowls, as I said, lay continually; but the pigeon, dove, trygon, and œnas lay twice a-year; and the pigeon ten times. The greatest number of birds lay in the spring; and some of them produce many young, and this in two ways; some producing their young often, as the pigeon; others producing many at a time, as the domestic fowl. All birds with crooked claws, except the cenchris,[189] lay but few eggs. This bird lays the most of any of its class; for it has been observed to produce four, and it even produces more. Some birds lay their eggs in nests; but those that do not fly, as partridges and quails, do not make nests, but lay their eggs on the ground and cover them over with rubbish. The lark and tetrix[190] do the same. [189] Falco tinnunculus. [190] Tetrao tetrix or Otis tetrix.