Volume 4 of consists of five chapters that review specific aspects of fly and mammalian development. In Chapter 1, Y. Mishina and R. Behringer discuss various aspects of Müllerian-inhibiting substance (MIS) in mammals, from a brief history of its discovery to recent studies of the MIS gene in transgenic and knock-out animals. In Chapter 2, C. Rushlow and S. Roth discuss the role of the -group genes in dorsoventral patterning of the embryo. In Chapter 3, M. Yip and H. Lipshitz discuss the terminal (asegmental termini) gene hierarchy of and the genetic control of tissue specification and morphogenesis. In Chapter 4, R. Bachvarova discusses induction of mesoderm and the origin of anterior-posterior polarity in the mouse embryo, using the frog embryo as a paradigm. In Chapter 5, P. Vogt discusses human Y chromosome function in male germ cell development.