The chicken eggshell (ES) consists of 95% calcium carbonate and 3.5% organic matter and represents the first physical barrier to protect the developing embryo. During the second half of development, calcium ions from the inner ES are progressively solubilized to support mineralization of the embryonic skeleton. This process is mediated by the chorioallantoic membrane (CAM), which is an extraembryonic structure that adheres to the eggshell membrane (ESM) that lines the ES. The CAM surrounds the embryo and egg contents by day 11 of incubation (ED11) and is fully differentiated and functionally active by ED15. We explored the morphological modifications of the ES, ESM and the CAM at these two stages by scanning electron microscopy, and observed that the ESM becomes progressively detached from the inner ES, which is degraded during embryonic development. Concomitantly, the CAM undergoes major structural changes, which include the progressive differentiation of villous cells that extend to reach the ESM and the ES. This study is currently being complemented by transcriptomic analysis of the CAM to identify the genes associated with changes in its structure and with ES demineralization at ED11 and ED15, in order to investigate our hypothesis that the CAM plays a major role in embryo defense (solubilization of ES antimicrobials/recruitment of immune cells) to compensate for eggshell weakening and increased susceptibility to pathogen attack from the external environment.