Reproductive Embryology, Anatomy and Physiology FORMATION OF THE AVIAN GONADS AND REPRODUCTIVE ANATOMY

FORMATION OF THE AVIAN GONADS AND REPRODUCTIVE ANATOMY The avian gonads arise from more than one embryonic source. The medulla or core arises from the mesonephric ducts. The outer cortex arises from a thickening of peritoneum along the root of the dorsal mesentery within the primitive gonadal ridge. Mesodermal germ cells that arise from yolk-sac endoderm migrate into this gonadal ridge, forming the ovary. The cells are initially distributed equally to both sides. In the hen, these germ cells are then preferentially distributed to the left side, and migrate from the right to the left side as well.58 Some avian species do in fact have 2 ovaries, including the brown kiwi and several raptor species. Sexual differentiation begins by day 5 in passerines and domestic fowl and by day 11 in raptor species. Differentiation of the ovary is characterized by development of the cortex, while the medulla develops into the testis.30,58