(A) Changes in the expression of sdf-1a and PGC migration. At stage 10, PGCs are identifiable via expression of the nanos gene. By stage 26, the sdf-1a expression domain has become restricted to the level of somites 11–13 in the lateral plate mesoderm. PGCs migrate to catch up with changes in sdf-1a expression and finally encounter the gonadal somatic mesoderm. (B) Formation of the gonadal primordium. Subpopulations of gonadal somatic mesoderm (GSM1 and GSM2) move medially to the prospective gonadal region, and begin to express sox9b and ftz-f1, respectively. The gonadal primordium separates into the left and right lobes at stages 33–35 (ventral view).