The tight aggregate elongates under control of the tip

The tight aggregate is at first a hemisphere, but soon elaborates a tip that becomes populated with prestalk cells. A large body of evidence indicates that prestalk cells move apically in the tight aggregate to establish the pattern, sorting out from the prespore cells as they go. The first sign of a tip is a signaling center in a mound, in which cells move radially around a center that does not yet form a morphologically distinct structure (Rietdorf et al., 1996; Siegert and Weijer, 1995; Sucgang et al., 1997). Often there will be more than one such center, but eventually, one predominates or, in large mounds, two tips form and the cell mass is subdivided. There is lingering confusion about what constitutes the tip because people use the word differently. In the text that follows, the tip refers to the most anterior part of the prestalk zone, which is usually suspended above the substratum. The word tip is also used to denote a signaling entity, and the morphological tip certainly fulfills such a role. We do not know whether the tip contains a specific group of cells, smaller than the physical structure, from which organizing signals emanate. The cells in the front of the slug are in constant motion, and it is hard to imagine how a few key cells at the tip could remain as a group.