The number of eggs laid in each clutch is constant for the individuals of the same species, but differs widely for different species. Many of the sea-going water birds lay but one egg a year— occasionally two. Those waterfowl classed as game birds lay from 10 to 14. The song birds and field birds generally lay four or five eggs in each dutch, but some few species do not lay more than two (as the pigeon) or three, while a few lay more — up to 10 or 12, or even 20, as is the case of the common partridge. Where two broods are raised in a year, of course the yearly production is doubled; and this, too, may happen in the case of birds which commonly rear but one brood, if the first nesting is destroyed by accident or otherwise. In several cases on record birds have laid the regulation clutch number again and again until exhausted, in the persistent endeavor to raise the yearly brood. The case of the common hen is in point: under natural conditions she would raise two broods of chicks during the year, from 13 to 15 eggs at a time. But under the exploitation of the poultryman, who removes the eggs from day to day, she continues laying for months at a time, producing up to 200 eggs per year.
Notable differences are found also in the texture and relative thickness of the shells of birds' egs. Some are so highly burnished as to resemble polished metal — as the tinamons. The eggs of kingfishers and of some wood peckers are glossy; those of ducks are waxen in appearance. Some are chalky and others are coated with a chalky crust of a different color from the hard smooth shell underneath —as with the cormorants and gannets, in which the egg is blue and the incrustation white. The shell of the ostrich's egg has the pecularity of being deeply pitted. Reference to the accom panying plate will give a clearer idea of the principles cited than can be conveyed by mere explanation. See also EGGS.
Eggs of Reptiles.— Reptiles, like birds, are distinctly oviparous, with few exceptions. Their eggs are, however, hatched by the heat of the sun, or of the fermentation of decaying vege table matter. In the few exceptional in stances referred to the young break out of the eggs as soon as they are laid, and in at least one instance — the stump-tailed lizard of Australia — the young complete their development and leave toe eggs before they are laid. Among the snakes, too, most of the boas bring forth their young alive, and so do some of the deep-sea snakes. An exception is to be noted in the case of the python, which lays from 15 to 100 eggs at a time and broods them until the young emerge.
The eggs of reptiles differ from those of birds in two respects — they are without a cal careous shell, having a parchment-like skin in stead, and in shape they are spherical or ellip tical, not ovoid nor conoid. In size they are
much smaller proportionally to the parent than is the case with the birds. The egg of the crocodile, for example, is about as large as that of a goose. Those of the giant tortoises of the Galapagos are spherical and about the size of hens eggs, and in contrast to those of other reptiles have hard calcareous shells. Croco diles as a rule lay their eggs in sand, in some cases digging out a pit two feet deep and
Eggs of Amphibia.— Among the frogs and toads the eggs are spherical, with a thin mem branous envelope, in which the embryo is dis tinctively visible surrounded with the yolk and with one or two outer coatings of a gelatinous substance. The latter is not noticeable when the eggs are first laid, but as they are laid in water the gelatinous substance swells into a relatively thick layer of slippery, non-soluble jelly. The eggs vary in size from one-twenty fifth of an inch (with the ordinary toad) to nearly one-half an inch in diameter with the Megalophrys longipes of the Malay Peninsula, the eggs of which are not laid in water but hid in damp moss, the little frogs emerging in a condition of complete development. The number of eggs laid by those frogs and toads which lay them in shallow pools is very large, reaching 12,000, and, as might be supposed, the destruction of these unprotected eggs in enor mous. Some species of tree-frogs lay their eggs in a mass of froth on the leaves of branches which overhang a pool. When the young hatch out they drop into the water to complete their development. These frogs lay about 200 eggs at a time. A small New Guinea frog lays 12 to 18 eggs enclosed in a small elliptical sack which is set adrift in the waters of a running stream. Several of the toads and frogs are noted for carrying their eggs about with them, in the water or on land, as the case may be, until they hatch.