Page:Natural History, Birds.djvu/14



numerous Class of vertebrated animals which we are about to consider, differs from that of the Mammalia, in that the young are not born alive, but are produced from eggs; which consist of a living point, attached to a globular sac of nutriment, called the yolk, surrounded by a layer of albumen, the glaire, and inclosed in two series of membrane, and a hard calcareous shell. For the development of the vital point into a living and active chick, it is needful that it should receive the stimulus of warmth; and this is, in general, supplied from the body of the parent bird, during the process of brooding or incubation; while it is retained, in most cases, by means of nests, in which the eggs are deposited, and which are composed of substances more or less calculated to resist the rapid abstraction of heat by the surrounding atmosphere. During the process of incubation, which lasts only a few weeks, the yolk is gradually absorbed into the body of the inclosed chick, forming its sustenance, until B