Secondary mesenchyme cells (SMCs), in addition to their apparent morphogenetic role during archenteron elongation, attach the tip of the archenteron to the region of the ectoderm that will ultimately form the mouth of the pluteus larva. Hardin and McClay (1990) showed that the behavior of the filopodia extended by SMCs changes dramatically when they strike a small region near the animal pole on the ventral (oral) side of the embryo. The average "lifetime" of a filopodium goes from roughly 5 minutes to as long as 1 hour. This raises the possibility that there is some information present in this region to which SMCs respond. The next several pages investigate this possibility.