Fig. 1. Early gonadogenesis in C. elegans. Somatic gonadal precursors (SGPs: Z1 and Z4), dark gray; primordial germ cells (PGCs: Z2 and Z3), light gray. (A) SGPs are specified within the mesodermal layer (white circles) and then migrate to meet PGCs. (B) SGPs and PGCs coalesce into the gonadal primordium, which at this stage has a left-right orientation. (C) During embryo morphogenesis, the gonadal primordium shifts to an anterior-posterior orientation, and acquires left-right and dorsal ventral axes. The first SGP division is asymmetric and segregates the potential to make two regulatory cells: anchor cells (AC) and distal tip cells (DTC). Genes crucial for early SGP divisions are noted.