BEHAVIOR OF EGG AND SPERM CENTRIOLES IN FERTILIZED-EGGS OF URECHIS-CAUPO

The origin of the cleavage centres in fertilized eggs of the echiuroid worm Urechis caupo (a protostome) was studied by examining the number of cleavage centres arising in artificially activated eggs, in polyspermic eggs, and in eggs in which the polar (meiotic) divisions were suppressed. The findings were supplemented by electron microscopic observations. The results show that, as in the deuterostomic echinoderms, egg centrioles are used to organize the meiotic spindles of the egg, but apparently stop functioning immediately afterwards. The centriole(s) introduced by the fertilizing sperm take over the function of spindle organization in the first cleavage division of the zygote.