H-Thymidine incorporation experiments in Barbus conchonius showed that presumptive primordial germ cells (PGCs) terminated their mitotic activity between midepibolys, and late epiboly. At the ten-somite stage, shortly after labeling of PGCs by uptake of H-thymidine became arrested, they could be recognized by their relatively large size and large nucleus. They were located in two longitudinal rows of cells between mesoderm and periblast, always at the same distance to the left and right of the notochord. Contact with the endoderm was not observed before the 16- to 23-somite stage. The numbers of PGCs were small (mean number, 18-19) and remained small for nearly 3 weeks. Mitotic activity was not observed in PGCs during that period; thereafter, rapid proliferation began. There is no evidence for active migration of PGCs; it is assumed that they are merely translocated passively together with their surrounding tissues. No specific constituents were detected with histochemical methods for glycogen, alkaline phosphatase, and RNA. Electron microscopy revealed the presence of "nuage" around the nucleus of PGCs. This material corresponded with perinuclear dense bodies as seen with light microscopy from the 19-somite stage onward. It is concluded that presumptive PGCs segregate from the somatic cells between midepiboly and late epiboly, before the three germ layers have been formed, and that locations of PGCs in the endodermal or mesodermal layer may be merely transitory stages during their translocation toward the gonadal primordia.
