In oviparous species, the egg contains protective systems and provides energy and nutrients that support the development of the embryo until hatching. During embryonic development, extraembryonic structures are vital to assimilate egg nutrients, to protect the embryo, to ensure gaseous exchange and to accumulate metabolic waste. The chorioallantoic membrane (CAM), which develops in close contact with the inner eggshell from day 5 of incubation, is a highly vascularized structure. The CAM is located between the eggshell membranes (ESM, outer side) and the allantoic fluid that collects embryonic waste (inner side). The CAM was reported to be involved in decalcification of the inner eggshell and mineral transport from the eggshell to the embryo (for mineralization of its skeleton), in acid-base homeostasis, and in ion and H2O reabsorption from the allantoic fluid. To decipher the kinetics of its structural maturation, we compared the structure of the inner and outer surfaces of the CAM by electron microscopy. In parallel, ESM and the eggshell were analyzed to better appreciate the impact of CAM development on its surrounding structures. Eggs from Ross 308 broiler hens were incubated for 11 or 15 days (ED11 vs ED15, early and mature stage of CAM development, respectively). At each time point, eggs were broken, and the eggshell, ESM and the CAM were gently separated for analysis by scanning electron microscopy. Images corresponding to the cross-section and the inner side of the eggshell, eggshell membranes (ESM, outer and inner sides) and CAM (outer and inner sides) were obtained. At ED15, results showed that the inner side of the eggshell is degraded (decalcification) during embryonic development while the mammillary knobs of the eggshell remain tightly attached to the external membrane of the ESM. The CAM also undergoes major structural changes with some villous cells that emerge from its outer surface to reach the eggshell membrane and the eggshell. The molecular components associated with eggshell decalcification and mineral transport from the eggshell to the embryo via CAM’s vasculature remain essentially unknown. This study has been recently complemented by a transcriptomic analysis of the CAM (RNA sequencing) at these two developmental stages. Altogether, we believe that the combined findings of this work will lead to better understanding of CAM functions that are vital for embryonic development and skeletal mineralization during incubation.