Fig. 3. Order of events during PGC specification. Comparison of the timing of different PGC events in C. elegans, Drosophila and mouse. For C. elegans (where PGCs do not migrate significantly), the `migration' period refers to the period from the birth of Z2 and Z3 to hatching. During this time, Z2 and Z3 associate with the somatic gonad. The `gonad' period refers to the period after hatching (L1 stage), when Z2 and Z3 start to proliferate.