ABSTRACT
Erythropoiesis during embryogenesis of the chicken, other avians, as well as the phylogenetically earlier submammalian vertebrates is conducted successively in different organs. The initial erythropoietic site in the avian (as in the mammal) is the yolk sac. And in the case of the chick and other avians, the yolk sac serves as the principal erythropoietic (and hemopoietic) organ during most of the prehatching period. The next loci to generate erythrocytes in the embryonic bird are the spleen and liver. Their contribution is limited in amount, transient in duration and seemingly quantitatively dependent upon the species as well as the individual animal. Though vigorous documentation is lacking, it seems that these latter organs may be uninvolved in erythropoiesis in certain cases. The onset of this activity in the spleen of the chicken is approximately incubation day 9, the peak is attained around day 14, and erythroproductivity persists until incubation days 16–18. Bone marrow erythropoiesis begins in the chicken on day ±12 of its prehatching period and continues thereafter to become the definitive permanent site of erythropoiesis (and granulo-thrombocytopoiesis) (Dieterlen-Lièvre 1988). In some species erythropoiesis also occurs in the liver and spleen of adults but only for limited periods (Romanoff 1960).
