method used to determine when the pigment precursor equivalence group is specified. Cytochalasin treatment of early tailbud stage embryos (17 hrs), while the two bilateral cells are still separated, results in both cells becoming ocelli. When the drug was used after the two cells aligned at the dorsal midline, the anterior cell developed into the otolith and the posterior cell became the ocellus without exception. Both experiments suggest that fates of the pigment precursor cells are irreversibly determined by approximately the mid-tailbud stage (21 hrs). Other Equivalence Groups Equivalence groups have also been described in the ganglion mother cells in grasshopper and the O/P teloblasts in the leech. Like other instances of equivalence groups, progeny cells are born equivalent and become specified through cell interactions. Equivalence groups are a common theme in the development of many organisms from diverse phyla. References External links Nematode Vulva Development Ascidian Network for In Situ Expression and Embryological Data Developmental biology