The structure of the hen's egg shel1 has been examined by many researchors under the optical microscope, polarization microscope, and electron microscope. In the present study, the structure of the egg shell of the White Leghorn hen was examined under the scanning electron microscope.
Completed eggs just after oviposition were used in this study. They were broken mechanical1 y with a knife, and the inside was washed shortly with a physiological saline solution. The shells were fixed in a 10% formalin solution. After fixation, they were calcified by 3% glacial acetic acid. They were also dipped into 30~35 % sodium hydroxide to remove organic substances from them.
The resulting materials were coated with gold or aluminum, and examined under the scanning electron microscope, Type JSM-2 (Japan Electron Optics Laboratory, Ltd.), at 25 KV accelerating voltage.
As is already known, the avian egg shell, from inside to outside, consists of the inner shell membrane, the outer shell membrane, the mammillary layer, the spongy layer, and the cuticle. By scanning electron microscopy, the inner shell membrane was divided into two layers, an inner and an outer layer. The inner layer of the inner shell membrane had an amorphous granular appearance and contained fine fibers. The outer layer of the inner shell membrane was formed by a meshwork of fibers. These fibers were 1~2μ in width and run parallel to the surface.They were surrounded by dusty substances that made their demarcation somewhat unclear.
The outer shell membrane was formed by a relatively dense meshwork of fibers. These fibers were 3~4μ in width and looked like tapes. They anastomosed with one another in such manner as to form knots. The outer surface of the outer shell membrane was not so sharply defined. Some of the fibers entered the calcified part of the mammillary knobs.
The mammillary layer was made of numerous conical mammillary knobs (mammillae) arranged regularly with their base on the spongy layer. The calculous mammillary knobs from which organic substances had been removed were 50~60μ in diameter at their base, and the diameter diminished toward the top. The top of each knob was somewhat flattened, and the center was crateriform (the central depression). Several small canals were open at the bottom of the central depression. The border of the central depression and the slope of the protrusions were indented sharply with many sulci. The individual knob had a large cavity, whose long axis was perpendicular to the surface. The cavity was filled with organic matrix of the natural egg. The mammillary layer changed gradually toward the spongy layer.
The spongy layer was densely calcified, yet the calcification was denser near the outer surface than in the deeper portion of the layer. On the other hand, the shell layer was constructed with dense and coarse calculous columns, which were arranged radially to the surface at regular intervals. In the clacified shell, these matrix fibers were 1~2μ in width and gave off fine branches. There was anastomosis among neighboring branches.
The outer surface of the shell was not smooth, and had a strikingly convexo-concave appearance. The concave area of the surface had contained air pores. Some air pores were open in the convex area. The air pores penetrated the spongy layer and opened at the interspace between the mammillary knobs.
The cuticle was a thin structureless membrane about 10μ thick.