ABSTRACT
Until recently, the cellular and molecular events comprising fertilization in birds were rather poorly understood. However, several recent studies in mammals as well as in birds have considerably contributed to our knowledge of the fertilization process. Mammals exhibit physiological monospermy, that is, only a single sperm enters the ovum. Monospermy is ensured by a variety of mechanisms, which is collectively known as the block to polyspermy. Unlike mammals, the principal feature of fertilization in birds, as well as in many fishes, amphibians and reptiles, is physiological polyspermy, that is, penetration of the ovum by many sperm. It has been suggested that polyspermy occurs in animals that produce large yolky (megalecithal) ova.
