1 Cell Lineage in the Development of the Leech Nervous System GUNTHER S. STENT, DAVID A. WEISBLAT, SETH S. BLAIR, and SAUL L. ZACKSON l. INTRODUCTION 1.1. Developmental Cell Lineages The intricate structure and function of the adult nervous system is the result of developmental interactions of factors both intrinsic and extrinsic to the embryonic neurons and their precursor cells. To fathom the mech- anisms underlying these interactive processes, a detailed knowledge of the course of neurogenesis at the cellular level is essential. Once such knowledge is available, specific and well-focused questions can be for- mulated at the biophysical, biochemical, or genetic levels. One key aspect of the process of neurogenesis at the cellular level is cell lineage, i.e., the embryonic lines of descent of various types of neurons. The importance of cell lineage for understanding developmental processes was realized over a century ago by C. O. Whitman. 1 On the basis of his studies of the development of leeches, Whitman put forward the idea, then quite novel, that each identified cell of the early embryo, and the clone of its GUNTHER S. STENT, DAVID A. WEISBLAT, SETH S. BLAIR, and SAUL L. ZACK- SON . Department of Molecular Biology, University of California, Berkeley, CA 94720. 1 N. C. Spitzer (ed.), Neuronal Development © Plenum Press, New York 1982