Birds are characterized by physiological polispermią , it means that many spermatozoa to penetrate the egg cytoplasm during normal fertilization , but only one of the male pronuclei formed participates in the formation of the zygote nucleus . However, all overtime degenerate male pronucleus during early cleavage . Has not yet been explained , what in fact happens to those overtime przedjądrzami and what mechanism is responsible for their degradation. Our results indicate that in the Japanese quail oocytes are DNase I and II , capable of degrading in vitro phage l naked DNA , and the DNA contained in the sperm of quail . We hypothesize that these enzymes can degrade DNA derived from supernumerary sperm penetrating the egg bird during polispermicznego fertilization , so they are one of the factors of late cytoplasmic block against polyspermy . The presence of large DNase activity in oocytes may also explain the difficulty in obtaining transgenic birds by injections of foreign DNA into the cytoplasm of the fertilized egg .