Literature on the structure and mechanical properties of the avian eggshell is reviewed and information on several aspects found to be incomplete or open to re-interpretation, especially on factors affecting hatching. Studies have been carried out on morphological changes occurring in the calcareous shell of the hen and quail egg as the embryo takes calcium from the shell during incubation. Shells from hatched eggs of a variety of species were also examined and the spherulitic nature of the mammillary layer is discussed in the light of these observations. A new interpretation of the relation of the mammillary layer to the mechanical properties of the integument is also proposed. In parallel, a survey was made of pipping and hatching methods in many species, involving studies of published information, laboratory and field observations, and analysis of available film. A classification of hatching strategies is put forward, based on the degree through which the chick turns in the egg during hatching climax. A mechanical-testing programme, much of which was designed to simulate the pipping and hatching processes, was carried out mainly on shells and membranes from hatched hen and quail eggs. The results demonstrated that the egg integument of the hen and duck could be classified as a brittle material while that of the quail, pigeon and starling could be regarded as much tougher. Acoustic-emission tests (which have not previously been used in egg-integument studies) confirmed this distinction. The different effects of moisture content on the two types of integument are also documented. A correlation between the spectrum of hatching techniques described and the degree of brittleness or toughness of the egg integument is made. Finally, the evolutionary significance of brittle or tough integuments is discussed.
|Date of Award||1980|