Scanning and transmission electron microscopy have been used to study the structure of the hen's shell membranes and their relationship to the shell and to the chorioallantoic membrane. We have confirmed previous observations that the fibres of the outer shell membrane are of larger diameter than those of the inner shell membrane, and that the fibres of the outer shell membrane extend into the mammillary knobs of the shell. The shell membrane fibres are arranged in layers parallel to the surface of the egg and there is no interweaving between the layers. Individual fibres are randomly orientated and may extend for distances of at least 25 μm. It is suggested that relative movement between the oviduct and the developing membrane is random in direction and location. Each fibre consists of a core with a covering cortex, but the idea that the core may consist of keratin is criticised. A limiting membrane separating the surface of the albumen from the fibres of the shell membrane is also formed from this cortex. During incubation the chorioallantoic membrane becomes pressed against this inner limiting membrane. No correlation could be found between the positioning of the mammillary knobs and the patterning of the shell membrane fibres. It is suggested that the positioning of the mammillary knobs reflects the pattern of certain secretory cells in the genital tract of the hen. No significant changes in structure of thickness of the shell membrane could be found during incubation. The tips of the mammillary knobs, however, become detached from the shell and remain adherent to the shell membrane. © 1969 Springer-Verlag.
