FRONT VIEW (A) AND SECTION (B) OF INFERIOR LARYNX OF PEREGRINE FALCON.
(After Macgillivray.)
The ducts from the urinary organs open to the exterior through the cloaca, into which, as already mentioned, the digestive tube also opens. The chief point with regard to the urinary secretion of birds is the fact that it is semi-solid, and that it contains a quantity of the substance known as uric acid. The kidneys are placed some way back and near the cloaca; they are set on either side of the spinal column, between the transverse processes of the sacral vertebræ, and are generally divided into three portions of greatly varying size. On their inner edge are given off the ureters, which pass on each side to enter separately into the before-mentioned cloaca.
The right ovary of birds is always atrophied, and it is in rare cases only that rudiments of it are found (namely, in the diurnal Raptores). The oviduct is a coiled canal, the lower portion of which has strong, muscular walls, while internally the characters of its surface vary according to the substance which the glands of different regions add to the descending egg. The right oviduct is not so completely atrophied as is the ovary of the same side. This duct opens into the cloaca through which the egg passes to reach the outer world; as further development is so largely independent of the mother, the female organs offer no peculiarities of arrangement, or complexities of structure.
All birds lay eggs, or, in other words, the born young are not carried about by the mother till the time of birth. The advantage of this to a flying animal is so obvious that we may pass at once to describe the egg of a common fowl. The shell, which consists of organic matter and lime-salts, is found to be formed of two layers; it is in the outer one only that pigment is found. Both layers are traversed by canals, through which air can pass only when the shell is dry; that is to say, the outer pores of the shell are closed under the influence of moisture. This may be seen by removing the outer layers, when air or water will pass in quite easily. These canals are said to be branched in the Ratite birds, and to be simple in the Carinatæ. The shell is lined by the shell-membrane, which, again, is made up of two layers. At the broad end of the egg these two layers are separated from one another, and so give rise to that air-chamber which is found in stale eggs, and increases in size as the egg grows older and the yolk evaporates.
The shell-membrane is in direct contact with the white of the egg (albumen). This, in its fresh state, consists of fluid albumen, arranged in layers, which are separated from one another by networks of fibres, in the meshes of which, however, fluid albumen is also to be found. There are, further, two special sets of fibrous cords in the white of the egg; these extend somewhat along the long axis of the egg, though they do not reach to the shell-membrane. From their bead-like character they are known as chalazæ (hailstones), but their more common English name is that of the “tread.”
DIAGRAMMATIC SECTION OF A FOWL’S EGG.
(bl) Blastoderm; (wy) White Yolk; (yy) Yellow Yolk; (vt) Vitelline Membrane; (w) Albumen; (ch) Chalazæ; (ach) Air-chamber; (ism) Internal Layer of Shell Membrane; (em) External Layer of ditto; (s) Shell.
The “white” is separated from the yolk by the so-called vitelline (or yolk) membrane; the greater part of this yolk is known as the yellow yolk, and is made up of minute albuminous granules, but its outermost part is formed of a thin layer of a somewhat different substance, which goes by the name of the white yolk. The spheres of this latter are still smaller than those of the yellow yolk, and they are also found to form layers at various levels in it. At one point the white yolk becomes a good deal thicker, and forms, as it were, a pad for a small white disc, which, in ordinary circumstances, is always found uppermost when an egg is opened. This disc is formed of an encircling white rim, and within it there is a rounded transparent region, the centre of which is more opaque.
This region is known as the blastoderm, and is that part of the egg from which the chick, with its organs and complicated vessels, muscles and bones is soon to be developed. In the laid egg, this blastoderm consists of two layers of cells, as do at a certain stage the eggs of all but the very simplest of animals. The dissection of a laying fowl will probably reveal the presence of eggs at an earlier stage, and from their study the following history has been made out: the ellipse-shaped egg, when about to leave the ovary, is a yellow body enclosed in a fine membrane, and possessing at one pole a small (germinal) disc; this disc contains a smaller germinal vesicle, and a still smaller germinal spot; when this body is ripe, it escapes from its enclosing capsule, and the germinal vesicle disappears. As the egg passes down the oviduct the albumen becomes deposited around it, and part of it is converted into the shell-membrane. The egg now becomes subjected to a thick, white fluid, which is gradually converted into the shell.