“Indent” Formation and Mitotic Delay of Sea Urchin Eggs Fertilized with X-irradiated Sperms

Abstract

When eggs of the sea urchin, Clypeaster japonicus, were fertilized with sperm irradiated with 4 kR X-rays, the first cleavage was delayed about 30 min. In addition, the disappearance of the indent and the end of the streak stage (marked by the disappearance of the nuclear membrane) were delayed by the same length as the first cleavage. From these observations, it is apparent that division delay induced by the irradiation is due to the prolongation of the streak stage.