In the leech, adult muscle cells and embryonic mesodermal/myogenic cells express the cell-type-specific Laz 10–1 epitope on extracellular matrix-associated proteins. Using this muscle-specific epitope as a marker, we found the following correlations between the development of identifiable myogenic cells at the embryonic midline and the segmentation of the leech CNS into 32 reiterative ganglia. (1) During the production of mesodermal and ectodermal stem cells, cell bodies of midline myogenic cells create 32 anterior-posterior intervals along the midline of the embryonic germinal plate. The mesoblasts then rearrange themselves into 32 somites whose spacing follows the intervals between the midline myogenic cell bodies. (2) Bilateral segmental zones of myogenic differentiation originate in juxtaposition to the midline myogenic cells. (3) The first two types of muscle precursors develop from midline and adjacent bilateral myogenic cells; they are the precursors of the CNS muscles and of three groups of ventral blood sinus muscles. These two types of muscle precursors demarcate the boundaries of the territory within which neuroblasts proliferate and coalesce into segmental hemiganglionic primordia: Cell bodies of muscle precursors, like cornerstones, demarcate the anterior and posterior borders of the hemiganglionic primordia; their longitudinal processes surround the expanding hemiganglionic primordia on the medial, lateral, dorsal, and ventral aspect. The contours of muscle precursors and midline myogenic cells are sharply delineated by immunohistochemical staining of the Laz 10–1 epitopes. In contrast, undifferentiated mesoblasts are surrounded by diffuse layers of stained epitopes, which are expressed at fluctuating levels. Because elevated levels of this matrix epitope are associated with mesodermal/myogenic cells undergoing morphogenetic rearrangements, it may participate in the molecular mechanisms underlying the segmentation of the nervous system.