CHAPTER XVIII.

FORMATION OF THE EGG AND CHICKEN.

THE reproductive organs of birds, and especially of the domestic fowl, are among the most complicated, since they must not only produce the ovum, but must enclose it in a protective envelope or sac, and supply nourishment for the embryo when growth subsequently takes place. As the ovum is developed after it is outside of the body, this elaborate protection is necessary in order to preserve it in its original form, to retard evaporation of water, and at the same time to permit the absorption of oxygen and moisture. The reproductive organs are composed of the ovary and the oviduct (Fig. 145). The oviduct is divided into two distinct parts, one secreting albumin and the other the shell.

The Ovary. The active ovary lies on the left side of the body posteriorly, close to the backbone. It resembles closely a bunch of grapes which vary in size from that of a walnut, when the ovum is fully developed, down to that of a pin-point. The ovum corresponds to the yolk or yellow part of the egg. All hens have at maturity a certain number of embryonic ova, which represent all the eggs or yolks which it will be possible for them to develop during life, the rapidity with which these will mature depending upon management and feeding. The development of the ovum is brought about through an excess of feed, that is, material beyond what is required for maintenance and growth; hence egg production cannot attain its maximum unless plenty of feed be given. By the accumulation of yellow or yolk the ovum is brought to maturity, the yolk sac or calyx distends, and, as development progresses, the yolk sac becomes detached from the ovary and completely covers the ovum. When the ovum reaches maturity, the calyx or yolk sac gradually recedes along the white stigma line, allowing the egg or yolk proper to slip from the capsule, at which time it drops into the expanded, upturned opening of the oviduct. The remaining calyx immediately contracts, rapidly diminishes in size, and is soon entirely absorbed.

The oviduct is a large tube, whitish in color, which starts from a point just below the ovary and ends at the cloaca. During the breeding season it becomes much enlarged, and is covered with a

network of fine blood vessels. The upper opening of the oviduct is expanded in the shape of a funnel to facilitate the entrance of the ovum when it emerges from the yolk sac. The duct on its

way to the cloaca lies in folds, being lapped upon itself three times before it finally passes into the cloaca.

When the yolk enters the oviduct it consists of a yellow mass, called the "vitellus," enclosed in a thin membrane, the "vitelline " membrane. Fertilization takes place in the upper part of the oviduct before any albumin has been added, and if at this time the

yolk be examined there will be found, just beneath the vitelline membrane, a small circular whitish body, about one-eighth of an inch in diameter, which will always float uppermost, no matter in what position the egg is laid. In the fertilized egg this disk has three well-defined areas,-an outer white rim, within which is a clear zone, and in the centre of this zone a somewhat elongated structure which is the embryo itself. These three parts constitute what is known as the blastoderm of the hen's egg, the part whose function is reproduction; all the rest of the egg is directly utilized for nutrition and protection.

The passage of the egg down the oviduct, from this time on, is marked by the following processes:

By the muscular contraction of the walls of the oviduct, the yolk is propelled to the cloaca. The passage of the yolk stimulates the walls of the duct to secrete successive layers of albumin. The first layer is thick, and has thickened portions which are fastened to the opposite poles of the yolk, to hold it in place. These thickened portions are called chalazæ; they are heavy twisted threads of albumin. After the thick layer of albumin two layers are added, each of which is thinner and more watery than the preceding one. When the egg reaches the lower or contracted part of the oviduct, two very tenacious but thin layers of albumin are added, to form the inner and outer shell membranes. The air cell, which is always perceptible at the larger end of the fully developed egg, is not present at first, but is formed by the evaporation of the fluids of the albumin and the entrance of atmospheric air to replace these fluids after the egg is laid. Just before the egg is discharged into the cloaca, it passes through a section of the oviduct where the shell is added, in the form of a coating of thin liquid secretion containing lime, which hardens quickly.

The perfect egg with its hardened shell is retained in the cloaca a short time before being laid. The power to hold the egg in the cloaca is under the control of the bird, so that, if she is frightened from her nest or unable to reach it, she can retain the egg for some time, the exact period varying with individuals.

The parts of the completed egg are the shell, shell membranes, albumin, and yolk (Fig. 146). The use of each is discussed in the following paragraphs.

The shell is the outer envelope of the egg, and its function is primarily that of protection. Soft-shelled eggs are those that have either no shell or else a very thin one. This is due to a deficiency

of lime salts in the feed, hence the necessity of supplying lime. It is estimated that one hen laying 100 eggs of average size will produce in a year about 22 ounces of carbonate of lime. The bird gets this large amount of lime from such materials as shells, dust, stones, and pieces of bone, which it constantly picks up when at liberty; if confined, oyster shell is given to supply the deficiency. The eggshell itself is composed chiefly of carbonate of lime, phosphate of lime, and a little animal matter. It is extremely porous, hence permeable by air, which is needed to supply oxygen to the embryo during incubation. Microscopic examination reveals thousands of

minute pores in the shell, by means of which evaporation is going on constantly from the time of laying. The degree of this evaporation is a sure indication of the age of the egg or the conditions under which it has been kept. Evaporation proceeds much more rapidly in warm weather than in cold, and in dry air than in moist; hence the difficulty of keeping eggs fresh for any length of time in summer. This time could be greatly increased if it were possible to close these pores or openings and thus stop evaporation, which, however, would make the egg unfit for hatching.

Shell Membranes.-Immediately under the shell are two membranes, called the inner and outer shell membranes. Both of these adhere closely to the shell, the outer one being in direct con

tact, while the inner one is adherent to the outer. At the large end, however, they are separate, and form a small cavity or sac known as the air cell. This is filled with air which has been drawn into the egg owing to the evaporation of the liquids, the size of the air cell increasing as the age of the egg increases. If this cell is found in any place but the large end of the egg, it usually denotes that the egg has been kept for some time, and has not been turned; thus the liquids settle and force the air to the highest point.

Albumin. Next is seen the albumin or white of the egg. This is a translucent liquid without definite structure, and, in the fresh egg, free from smell and taste. The albumin is easily coagulated by heat, only about 140 degrees being required. It is soluble in water. Under chemical analysis albumin is rather complex, being composed of varying proportions of oxygen, hydrogen, sulphur, phosphorus, and carbon. The albumin can be readily divided into three distinct layers; a thin outer coat, a thicker middle layer, and a very much thickened inner stratum. In the latter can be found the chalaza, which are thickened strings or cords of albumin, having an outer terminus in the outer layer of albumin, and an inner securely interwoven in and around the yolk; they run lengthwise of the egg, are twisted in opposite directions, and hold the yolk in place. In appearance they resemble twisted cords. The albumin has two distinct functions; the first is that of protection, by forming a thick layer of material around the germ, at the same time that it furnishes the material in which it floats, thus protecting it from shocks which otherwise might prove fatal. The second purpose is that it provides material which goes directly into the formation of tissue for the embryo.

The Yolk. The yolk or ovum may be called the most important part of the egg, since it contains within its outer wall, or vitelline membrane, the germ or embryo and also the nourishment of the growing embryo. The yolk is nearly round, and light yellow in color, the degree or intensity of color varying with the character of the feed. In composition the yolk is considerably richer than the albumin, especially in phosphate of lime, there being but a trace in the albumin. The yolk is composed of two distinct portions arranged in concentric rings or layers, one being light in color and not coagulable upon the addition of heat, the other darker yellow in color and easily coagulable at a high temperature. In the lighter portion of the yolk is a hollow or indentation, in the upper part of which, and next to the vitelline membrane, lies the