Spicule Formation‐Inducing Substance in Sea Urchin Embryo

Isolated micromeres of sea urchin produced spicules in sea water containing blastocoelic fluid (BCF) taken from embryos, or in a medium in which embryos had previously been dissociated (dissociated solution, DS). When isolated micromeres were cultured in vitro, their descendants initiated spicule formation only when BCF was added to the culture medium by the time when, in normal development, primary mesenchyme cells form two aggregates in the vegetal region. After the initiation of spicule formation, growth of spicules occurred under the continuous influence of DS. Spicule formation-inducing (SFI) activity in DS was first detected at the mesenchme blastula stage. The activity in BCF was heat-labile and was inactivated by trypsin.