initiated) were collected as donors and hosts, respectively. Arrhenogenic females (series A) and thelygenic females (series B), both of them carriers of w/w, served as donors while the hosts consisted of aand t-type individuals phenotypically identical and with wild-type eyes ( + / + ) . Each of the hosts was provided with one additional ovary. In some of the hosts the implanted ovary grows normally, establishes contact with the host's own oviduct, and undergoes typical differentiation (Fig. 1). Crossings between such host females and w/w males reveal whether or not, apart from the eggs developed in both of the host's own ovaries and giving rise to adults with wild-type eyes (+/w), there also appear female and/or male offspring with white eyes stemming from the eggs of an implanted w/w ovary. In both of the experimental series (A, B), from a total of 481 operated recipient females, 27 (5.6%) produced offspring from the hosts' own ovaries as well as from the implanted ones. The offspring of 13 of them consisted of individuals of both sexes (Table 1). The appearance of females and males among the offspring from originally either thelygenic or arrhenogenic female hosts each carrying an ovary implanted from an aor t-donor, respectively, indicates that individuals from the eggs that have developed in the implanted gonads are always of the sex corresponding to the genetic constitution of the donor in question, independently of the thelygenic or arrhenogenic character of their female hosts. If the effective predetermining gene product were synthesized within somatic cells, i.e., outside the ovaries, it would have to have been produced and transported to the cells of the germ line before the developmental stage chosen for transplantation. Further transplantations at still earlier ovarian stages and others to be carried out with pole cells, whereby the role of the somatic cells of the ovary may be tested as a possible site of synthesis of the F gene product, shall help to solve this problem definitely. Supported by the Deutsche Forschungsgemeinschaft; the author thanks Miss M. Vogel for excellent technical assistance.