The development of the embryo does not always take place immediately after the egg is laid, and a considerable time may elapse before it commences. Thus, the first eggs laid by the bird do not to undergo development until the whole number which is to constitute the entire brood is deposited; and the eggs of most insects are laid in the autumn, and remain unchanged till the spring. During this the vital principle is dormant. See DORMANT VITALITY. 111 the case of birds and tiles, a considerable degree of warmth is requisite for the process of development. In birds, the act of incubation, or "sitting," supplies the necessary amount of heat; and in' reptiles, the eggs are hatched by exposure on sands to the sun's rays, by their being deposited in warm dunghills, etc.
In the composition and structure of a newly-laid lien's egg—and of course the description equally applies to the egg of any other bird—we have exteriorly the shell, whose hardness is due mainly to carbonate of lime. and which is lined by a double membrane, the shell-membrane; then there is an albuminous substance, the white, iir which several layers may be distinguished; within this, we find the yolk, inclosed in its membrane; and in the midst of the yolk there was, before it was laid, a minute vesicle, the germinative vesicle, containing a still smaller one, the germinative dot. The most con spicuous of these parts—namely, the shell and the white—are not physiologically essen tial, and are wanting in the eggs of most animals lower than birds; while the yolk, the germinative vesicle, and the germinative dot, are found in the eggs of all animals; and it is from these parts, and these only, that the germ is formed. The yolk or vitellus is the most essential part of the egg. It is a viscid fluid, sometimes opaque and colored, as-in the eggs of birds, and sometimes transparent and colorless, as in the eggs of fishes and mollusks. The microscope shows that it is composed of an accumulation of granules and oil-vesicles. The yolk is surrounded by a very thin skin, termed the vitelline membrane. In some insects, this membrane forms the outer coat of the egg.
The germinative vesicle is a cell, situated in the young egg near the middle of the yolk; it contains one or more minute spots or nuclei, termed the germinative dots, which themselves contain smaller nucleoli. The albumen or white of the egg is a viscid and colorless fluid; which coagulates and becomes opaque on the application of a temperature of 146° F. Although it forms a large part of the egg of the bird, it is of very trifling importance in reference to the development of the embryo. It is not formed in the ovary like the yolk, but is secreted by the oviduct, and deposited round the yolk during the passage of the egg through that canal; hence, when there is no oviduct, the albumen is geners ally absent. In birds, it consists of several layers, one of which, forming the chalazd, presents a coiled, rope-like appearance. The albumen in this part is thicker than that which lies more externally. The albumen is bounded externally by the membrana puta
minis, or Melt membrane, which splits into two layers, leaving a space at the broad end, filled with air; in birds, and in some reptiles, this membrane is covered by the calcar reous shell; in most cases, however, it continues membranous! as in the eggs of the mob tusks, most crustaceans, and fishes, frogs, &c.; and sometimes it is horny, as in the sharks and skates.
In order to understand the successive steps of embryonic development, we must remember that the whole animal body is formed of tissues, time elements of which are cells. See CELLS. While in the full-grown animal these cells have undergone such modifications as often hardly to present any indication of their primary form; in early embryonic life, the cells, which originate within the yolk, present a definite form and consistence. These cells we shall presentlysee become transformed into the blood, bone muscle, etc., of the young animal. At the commencement of the process of develop ment, the yolk, which previously was a mass of uniform appearance, gradually presents certain alterations. Some portions become more opaque, and others more transparent; and time germinative vesicle, which was in the center, rises to the upper part of the yolk; where the germ is to be formed. The yolk at the same time undergoes a peculiar pro cess of segmentation. It is first divided into halves, forming distinct spheres, which are again continuously and regularly subdivided into two. till the whole or a part of the yolk assumes the appearance of a mulberry, and in known as the "mulberry mass." Id fishes, the class of animals 'Whose development we shall specially follow in this article, the segmentation is only partial, the divisions of the yolk not extending into its whole mass. ThiS process leads to the formation of a germ, which usually rises above the yolk in the form of a discoid protuberance, which has received the various names of germinative disk, proligerous disk, germinal membrane, etc., and which is composed of minute cells. This disk enlarges till it embraces the whole, or very nearly the whole, of the yolk. At this early period, the germ consists of a single layer of very minute cells of uniform appearance; it soon, however, increases in thickness, and in vertebrate ani mals separates into various layers, which gradually become more and more distinct. The upper layer is termed the serous or nervous layer, and from it are subsequently developed the principal organs of animal life—such as the bones, muscles, brain and nerves, etc. The lower layer gives origin to the organs of vegetative life, and especially to the intestines and chylopoietie viscera, and is called the mucous or vegetative layer; the cells of which it is composed are larger than those of the serous layer. Between these layers there is, in the vertebrata, an intermediate layer, giving rise to the organs of circulation and to the first-formed blood, and hence termed the vascular layer.