Fig. 5. PGCs during embryogenesis. (A) Spadella cephaloptera: sequence of four images showing the beginning of gastrulation. The first two PGCs are identified by the continuous presence of large interphase nuclei (colored ochre). During gastrulation these cells are the first to invaginate with the archenteron (arrows in second image) and to position themselves at the tip of the archenteron (last image of the sequence). (B,C) Sagitta inflata: late gastrula. The two first PGCs have divided once. They are identified by their well-delineated interphase nuclei. (D) Sagitta bipunctata: as the trunk of the embryo elongates, PGCs migrate (three PGCs are visible due to their large interphase nuclei). (E) Sagitta bipunctata: thin section through one of the PGC at gastrula stage (see sequence A). Many electron-dense islands (arrow) are dispersed in the cytoplasm. N, nucleus of the PGC. (F) Sagitta bipunctata: median portion of a juvenile showing the four PGCs positioned in pairs above and below the transversal septum (TS).