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The aves such as chickens, quails and pheasants have unique characteristics, which is based on its evolutional position between mammals and other species. Ex vivo development of the aves is the most prominent difference from the mammalian species which undertake in vivo organogenesis and relevant or addition difference are found during embyogenesis and organogenesis. Differing from mammals, physiological polyspermy is usually induced at the time of fertilization and numerous sperms are visible, regardless of embryonic nuclei formation. Consequently, asymmetric cleavage yielding pre-blastodermal cells is observed throughout early embryogenesis and the intrauterine eggshell formation consisting of three stages is underwent before laying. According to developmental stage, the eggs consisting of 1 to 3,000 cells form yellowish soft eggshell membrane at the phase I. Phase II eggs consisting of 3,000 to 30,000 cells have light yellowish, flexible eggshell and phase III eggs consisting of 30,000 to 60,000 cells have milky white stiffened eggshell. Such difference from the mammals may induce unique cell fate determination in various systems, which may be prominent in mesenchyme-derivative tissues and organs. [ABSTRACT FROM AUTHOR]