There's something about the midline

Primordial germ cells (PGCs) arise in a single cluster and then migrate to the two gonads during embryogenesis. Sano et al. (page 675) find that in flies the previously known guidance factors are not required for the process. Rather, a lipid phosphate phosphatase that is expressed in the central nervous system (CNS) repels the migrating PGCs, pushing them along their proper route.

Previous work using fixed material showed the general path that the PGCs take as they move to the gonad. However, the process itself has been hard to piece together from still images.

Using videomicroscopy, Sano et al. saw that the PGCs leave the gut, moving from the middle of the embryo where they are formed. The cells travel up toward the dorsal side, where the CNS is, and then laterally...