The morphological mechanism of the shell formation of hen's eggs was observed in developing eggs in the uterus by scanning electron microscopy. The shell formation was started in the egg just after the egg had arrived at the uterus. The first indication of the shell formation was the deposition of small sand-like granules on the shell surface. The subsequent indication of the shell formation was the appearance of small-sized organic protrusions on the shell surface. These protrusions increased in size as the shell formation advanced and formed nuclei of calcification. The deposition of calcium salts progressed around the organic protrusions, and mammillary knobs were formed. Previous organic concretions were encrusted by the deposition of calcium and became a central organic core, which was located at the center of the mammillary knob. Mammillary knobs were first small in size and later increased in size progressively. At last, they fused with one another to form a mammillary layer. After the formation of the mammillary layer, a spongy layer developed successively on the mammillary knob. It became thick with additional deposition of calcium salts in a radial direction.