Later stages of sperm penetration and second polar body and blastodisc formation in the egg of Fundulus heteroclitus

Abstract

AbstractThis study represents a follow‐up to earlier papers in which the initial stages of sperm penetration and cortical vesicle breakdown in activated eggs of Fundulus heteroclitus were described. Light and electron microscopy were used to examine later events of fertilization, second polar body formation, and changes in animal pole cytoplasm as the embryonic blastodisc forms. Fusion of sperm and egg plasmalemmae is quickly followed by the formation of a distinctive fertilization cone which persists for only 3 to 5 minutes. The protruding sperm flagellum disappears to the inside of the egg within 10 to 15 minutes. Inside the egg cytoplasm the sperm nucleus begins to undergo chromatin decondensation and nuclear envelope fragmentation as early as 20 seconds post‐insemination. In the egg the second meiotic spindle underlies the first polar body and the second polar body begins to form within about 3 minutes post‐insemination. The animal pole cytoplasm triples or quadruples in thickness within 3 minutes after insemination. It continues to thicken to form the definitive blastodisc by 1 to 1.75 hours after insemination. Details of changes in the surface topography of the egg which accompany these events are presented.