Nuclear events from fertilisation to the early cleavage stages in the domestic fowl (Gallus domesticus)

The changes in nuclear morphology from fertilisation to the 16-blastomere stage of development in the domestic fowl were examined in sections of the germinal disc stained with DAPI (4',6'-diamidino-2-phenylindole) or with the Feulgen technique. By one hour after the estimated time of ovulation, the second meiotic division of the ovum was completed and the spermatozoal heads (3-9) in the germinal disc had transformed into pronuclei. During the following two hours all the pronuclei enlarged, and two of them became juxtaposed. Mitotic figures and large pronuclei were observed at 4 hours. The zygote nucleus went through 3-4 division cycles in synchrony. The supernumerary male nuclei divided once, at most, and all showed evidence of degeneration by the 4-blastomere stage, at about five hours. Therefore, the nuclei destined to form the future chick embryo follow a pattern of behaviour during fertilisation and early cleavage similar to that of animals in general.