LOCATION AND ULTRASTRUCTURE OF PRIMORDIAL GERM CELLS (PGCs) IN AMBYSTOMA MEXICANUM

The location and ultrastructure of the primordial germ cells (PGCs) were studied in Ambystoma mexicanum larvae of stages 23 to 47. PGCs were found in the spaces between the endodermal cell mass and the lateral plate mesoderm at stages 23 to 35. Some of the PGCs at stage 35, and most of them at stages 40 and 42, were located near the Wolffian duct. At stages 46 and 47 all the PGCs were situated in the genital ridges. Cilia, which have hitherto never been reported in PGCs, were occasionally seen in PGCs of Ambystoma from stage 23 till stage 46. No "germinal plasm" was found in the PGCs prior to stage 40. Specific structures or "nuage material", corresponding to the germinal granules or their derivatives in Xenopus, were first recognized in the vicinity of the nucleus at stage 40. Between stages 40 and 46, the amount of "nuage material" markedly increased. It was finally localized mainly in "intermitochondrial spaces". A possible transfer of material from the nucleus to the cytoplasm or vice versa through nuclear pores was first noticed at stage 40, the material concerned being quite similar in ultrastructure to the "nuage material".