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.[6] 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.