Comparative development of the turkey and chicken embryo from cleavage through hypoblast formation

The development of the turkey and chicken embryo from the first cleavage division through hypoblast formation is described. The early development of the chicken embryo has been categorized into 14 stages. A similar staging sequence for the turkey was not proposed until 1993, when we described the early development of the turkey embryo, which was divided into 11 stages. Comparatively, differences in the temporal and spatial development of the turkey and chicken blastoderm were evident. Of significance is the observation that at oviposition the turkey is in Stage VII and characterized by the first signs of area pellucida formation. In contrast, the chicken embryo at oviposition is in Stage X and area pellucida formation is completed. Similarly, the hypoblast, which is already apparent in the Stage X chicken embryo, does not appear in the turkey embryo until the egg is incubated. Furthermore, the anterior-posterior (head-tail) axis in the early embryo is achieved prior to oviposition in the chicken but after the onset of incubation in the turkey. It is apparent that the turkey embryo is less mature than the chicken embryo at oviposition. Whether this distinction is related to differences between the hatchability of turkey and chicken eggs is not yet known.