Primordial germ cells (PGCs) are undifferentiated germ cells in developing fetuses. As these cells give rise to definitive oocytes and spermatozoa that contribute to new life in the next generation, their development must be under strict control, regarding genetic and epigenetic aspects. However, we do not know to what extent their development depends on the specific milieu. In this study, we transplanted mouse PGCs collected from male and female gonads at 12.5 days postcoitum, together with gonadal somatic cells, under kidney capsules of adult mice. The transplanted PGC and gonadal somatic cells constructed testis-like and ovary-like tissues, respectively, under the kidney capsules within 4 wk. Normal-appearing round spermatids and fully grown germinal vesicle (GV) oocytes developed within these tissues. Ectopic spermatogenesis continued thereafter, while oogenesis consisted of only a single wave. The injection of these round spermatids directly into mature in vivo-derived oocytes led to the birth at term of normal pups. PGC-derived GV oocytes were isolated, induced to mature in vitro, and injected with normal spermatozoa. The injected oocytes were successfully fertilized and developed into normal pups. Our findings demonstrate the remarkable flexibility of PGC development, which can proceed up to the functional gamete stage under spatially and temporally noninnate conditions. This transplantation system may provide a unique technical basis for induction of the development of early germ cells of exogenous origins, such as those from embryonic stem cells.