The three-dimensional structure of the hen's eggshell was studied by scanning electron microscopy. The morphological subjects of this study included the process of formation of the eggshell, the fine structure of the mammilla and air pore, and the morphological changes of the hatched eggshell.
The first step of shell formation began in the uterus of the oviduct with the formation of small organic concretions on the shell membrane. After these concretions had grown to some extent, they were encrusted with the deposition of calcium and became cone-shaped mammillae. They enlarged gradually upwards and in the lateral direction, while they were prevented from growing inwards, because the site of their presence had been mineralized early. The mammillae were enveloped by a cuticle-like covering. The enlargement of the mammillae proceeded by alternative formation of the covering and the subsequent mineralization of this covering. Finally, the mammillae were fused with one another to form a single layer in the mammillary stratum. The completed mammillae rested on the shell membrane, with their tips embedded in this stratum. They were connected firmly with the shell membrane by means of fibers derived from its outer layer. The mammillary core was connected loosely with the rest of the mammilla by a small amount of fibrils. The liberation of the shell membrane from the shell during the incubation period took place at the boundary between the core and the rest of the mammillae. It was considered to be induced due to the loose connection at this boundary.
After the completion of the mammillary layer, spaces varying in size and form were left behind in places where mammillae had come together. These inter-mammillary spaces seemed to play a certain role in the formation of a spongy layer. This laver was formed on the mammillary layer by the superimposition of new calcite layers. The process of calcification of the spongy layer consisted in two phases. The first step was the formation of a network of matrix fibers, and the second one the cementation of the meshes of the network with calcium.
The air pores of the eggshell showed themselves as mushroom-like projections in the resin cast. They arose from the large intermammillary spaces and opened in shallow grooves on the shell surface. In addition to true air pores, there existed a number of blind or aborted canals which had been closed on the way of the entire thickness of the shell. These canals arose from small intermammillary spaces. The formation mechanism of the air pore was discussed.