A detailed examination of the developmental features of abnormal formation of pole cells and a functional analysis of the germ plasm of gs(1)N441 embryos were carried out. The germ plasm is morphologically normal. Embryos in which cleavage nuclei show retarded migration to the posterior pole do not form pole cells. Pole cells, following formation, are abnormally segregated and then intermingled between the blastoderm cell layer but retaining normal morphology and differentiating into functional germ cells. The results of cytoplasmic transplantation experiments indicate the autonomous segregation ability of the mutant polar plasm to form pole cells to possibly be affected.