  Three-dimensional random tessellations that are stable under iteration (STIT
tessellations) are considered. They arise as a result of subsequent cell
division, which implies that their cells are not face-to-face. The edges of the
cell-dividing polygons are the so-called I-segments of the tessellation. The
main result is an explicit formula for the distribution of the number of
vertices in the relative interior of the typical I-segment. On the way of its
proof other distributional identities for the typical as well as for the
length-weighted typical I-segment are obtained. They provide new insight into
the spatio-temporal construction process.
