The mammalian testis is a complex organ composed of multiple cell types that are organized into seminiferous tubules and an interstitium, producing spermatozoa and hormones, respectively. During embryogenesis, the testis forms from the genital ridge associated with the embryonic kidney called the mesonephros. After germ cells migrate into the genital ridge, the Sertoli cell-germ cell mass forms and undergoes morphogenetic changes to generate testis cords, the precursors of the seminiferous tubules. Static images of the fetal testis at sequential stages of development provide structural information about cord formation. Transgenic mice that express fluorescent protein reporters offer new opportunities for time-lapse imaging to visualize live cells and their behaviors during testis differentiation and morphogenesis.