Movie 1.

Serial transmission electron microscopy images from 90 nm-thick sections of the primordial germ line from a first larval stage animal. The bottom panel is a color-overlayed version of the top panel, depicting nuclei (blue), PGC cytoplasm (dark red) and cytoplasm within the membrane-dense structure between the PGCs (i.e. the proto-rachis, light red). The nuclei of the somatic primordial gonad (Z₁ and Z₄) are depicted in green. Images were aligned manually.