Part 60

The shell consists of several layers. Three are easily distinguishable: first, an inner mammillary layer, consisting of minute conical deposits of calcareous material; second, a middle spongy layer, composed of a thick network of fibers; third, an outer delicate, cuticle-like structure. In certain breeds of poultry a pigment may be added; for example, in ducks a pea green, in turkeys a spotted brownish material, and in fowls pink and various shades of brown. The egg shell is porous to admit the free exchange of air during the process of incubation.

An average sized hen egg weighs about 2 ounces, of which 11 per cent. is shell, 32 per cent. yolk, and 57 per cent. white. The principal chemical constituents of the egg are as follows: ash, or mineral matter, 9 per cent.; fat, or hydrocarbon, 9.3 per cent.; proteids, or nitrogenous matter, 11.9 per cent.; and water, 65.5 per cent. There is apparently no constant proportion of weight between the yolk and albumin. There is also a variation in the weight of the shell due to its variation in thickness.

In an examination of ten eggs of average size, the yolk constituted 31 per cent. of the total weight of the egg.

The following is the result of the analysis of twelve eggs of average size. This analysis included the shell and all other parts taken together.

Footnote 6:

The analysis was made by D. M. McCarty, chemist, Animal Industry Division, North Carolina Experiment Station.

Moisture 64.25 per cent. Dry matter 35.75 per cent. ———————————————— 100.00 per cent.

Parts per hundred including shell

Protein 10.2500 Fat 10.6200 Phosphorus .3020 Calcium .6080 Magnesium .0985 Iron .0103 Sulphur .3950 Chlorine .1506 Potassium .0103 Sodium .2000

=The Oviduct.=—The three separate and distinct portions of the egg, albumin, shell membranes, and the shell, are constructed in different parts of the oviduct.

Location.—The oviduct of the hen extends along the left side of the bodies of the vertebræ, and the roof of the pelvic cavity and lies dorsal to the abdominal air-sac. It extends from the posterior border of the ovary and empties into the cloaca (Fig. 56, No. 3 and 8) through a transverse slit.

Shape.—In a well-developed Plymouth Rock pullet, but one whose reproductive organs have never become active, the oviduct is about 4½ inches long; in the fully developed and active state it is from 18 to 20 inches long, and in a collapsed state about ½ inch in diameter. It is held in position by two ligaments, a dorsal and a ventral (Fig. 55, B) to be described later. The oviduct is tortuous in its course, forming three principle convolutions before reaching the cloaca, and like the ovarian mass, in its active state, pushes the abdominal viscera downward and toward the right side.

Structure.—The oviduct consists of three coats: first, the serous, located on the outside, which is a reflection of the peritoneum; second, the muscular middle tunic; third, the mucous coat, which in a resting state is thrown into folds.

FIG. 59.—The active oviduct of a hen laid open. 1, The ovary. 2, The funnel. 3, The albumin-secreting portion. 4, The isthmus. 5, The shell=gland portion. 6, The vagina. 7, The superior ligament. 8, The inferior ligament. ]

=Parts of the Oviduct.=—The parts of the oviduct are as follows: the funnel, the albumin-secreting portion, the isthmus, the shell-gland portion, or uterus, and the vagina (Fig. 59).

FIG. 60.—The mucous lining of the oviduct.

A. Transverse section of the oviduct wall in the region of the neck of the funnel showing primary and secondary folding of the epithelium (after Surface).

B. Showing the type of gland cells of the funnel region.

C. Transverse section through the wall of the uterus showing the deep folds.

D. Section from the albumin-secreting portion showing the opening of a tubular gland and also showing the character of the cells. ]

The funnel is the trumpet-shaped portion, the ostium tubæ abdominale, whose mouth or fimbriated opening faces the ovary, and lies ventrally to receive the ovum, or yolk, as it is discharged from the ovary. Its thin wall, expanded in the anterior portion, is provided with fimbriæ-like projections. This funnel-shaped portion soon converges to form a constricted portion. This portion of the active oviduct is from 3 to 4 centimeters long. The mucous membrane occurs in folds forming low longitudinal spiral ridges (Fig. 60, A). The fimbriæ are continuous with the dorsal and the ventral ligament of the oviduct; and from this point, where the ridges of the mucous membrane are almost nil, they gradually increase in height as they extend down the tube. These ridges are continued in those of the second, or albumin-secreting portion (Fig. 60, D). At this point they increase in height very rapidly. Here the bundles of muscular fibers of the middle coat are thin and distributed among bundles of connective tissue. The muscular fibers consist of two layers, an outer longitudinal and an inner circular. At places in this portion the inner bundles may be noted to extend longitudinally. In embryological development the epithelial layer has an origin different from the outer layers of the oviduct. In the fetal development the Müllerian duct arises as a thickening along the Wolffian body just ventral to the gonad. This Müllerian duct is at first a solid cord of cells. It later develops a lumen, and grows posteriorly until it connects with the cloaca. At the time of this posterior growth, mesenchyme cells migrate in from the surrounding tissue and form a layer about the duct. From this layer of mesenchyme cells there are developed the outer layers of the oviduct, which layers later develop the muscular structure and the connective tissue. The epithelium and its derivatives, which represent the glandular structures, are formed from the walls of the old Müllerian duct. Thus the two sets of tissues, having different origins, likewise have different functions. The epithelium is concerned entirely with secretion, and the derivatives of the mesenchyme are concerned with supporting and muscular function.

FIG. 60. A.—The mucous lining of the oviduct.

E. Section of the epithelium from the vagina showing unicellular glands.

F. A section from the isthmus showing opening of a tubule. ]

To summarize, six layers of tissue occur in the funnel region, namely, an outer serous covering, an outer longitudinal muscular layer, a layer of connective tissue, an inner circular layer, a second layer of connective tissue, and an inner mucous layer. The mucous layer is made up of glands, as follows. The unicellular glands occur between the ciliated cells of the epithelium. These glands are found only in the posterior half of this division of the oviduct. The glandular grooves are made up of an accumulation of gland cells at the bottom of the grooves between the secondary folds of the epithelium. These are found in all parts of this division except the extreme posterior part. In the posterior part we find the third type of glands, the tubular variety.

The second division of the oviduct, as stated above, is the albumin-secreting portion. The funnel division gradually merges into the second portion. These two portions are distinguishable from each other. The walls of the albumin-secreting portion are much thicker and the longitudinal ridges are higher. This section is the longest of the five divisions, measuring from 40 to 42 centimeters in length, or more than half the length of the oviduct. The albumin division terminates rather abruptly into the third division, the isthmus (Fig. 59, A).

It is probable that the secretion of albumin is not confined to the cells of the second division; yet we are safe in saying that the major portion is formed here. The folds of mucous membrane in this division are thicker and higher than in the funnel, due to their containing cells of the high columnar type, and to the fuller development of the glands which are of the tubular variety. The muscular layer is heavier, and therefore the muscular power to force along the tubes’ contents is greater. In the formation of the mucous folds we find, in transverse sections, that the central core is made up of connective tissue which carries blood-vessels and nerve filaments, as in other similar glandular structure. The epithelium contains glandular cells of two varieties, namely, the ciliated, columnar variety, and the unicellular, goblet variety (Fig. 36, No. B, 2). These two kinds of glandular cells are rather evenly distributed throughout the epithelium of this section of the oviduct. The unicellular gland cells are more numerous at the mouths of the ducts leading from the tubular glands. The nuclei of the ciliated cells are oval and lie near the middle of the cells or a trifle toward the base from the middle. The protoplasm of the cells is finely granular. Strong cilia in considerable number surmount each cell. In some cases the goblet or mucous cells have pushed apart the ciliated cells, and their prolongations extend farther than the surface of the ciliated cells. The nuclei of the goblet cells are round, and lie nearer the proximal end than those of the ciliated cells.

The third division of the oviduct, the isthmus, continues from the albumin-secreting portion and terminates in the expanded portion called by some anatomists, the uterus. Toward the posterior end of the albumin-secreting portion the longitudinal folds of mucous membrane become lower, making, at the juncture of this and the isthmus, a clear fine of demarcation. For a distance of 2 or 3 centimeters the folds are low, after which they gradually become higher, but never reach the height or thickness of those in the albumin-secreting portion.