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The shell of an egg contributes to successful embryogenesis in many ways such as protection, respiration, water exchange etc. The shell is also the major source of calcium for the development of the nervous system, muscles, and skeleton, i.e. the main source of calcium for the development of highly calcium consuming organs. Some studies suggest, moreover, that growth rate may play a fundamental role in the pattern of skeletal development of birds: the faster the growth the less ossified the skeleton becomes. We predicted, therefore, that fast- (altricial) and slow- (precocial) growing bird species should lay eggs encased in shells with different structures adapted to support different rates of calcium removal by the developing embryos. We tested this prediction by comparing the fine structure of the inner shell surface (mammillary layer) of eggs from birds with very different growth rates and modes of development (e.g. from Struthinoformes to Passeriformes) and obtained results in agreement with the prediction.