48 h old testis were implanted into different adult hosts. In virgin females the germ cell multiplication is lower than in both mated females or in males. However in all host types the differentiation into spermatids beginns at the same time as in situ.The role of the different adult host glands in male germ cell development was investigated. Through inactivation of such glands in mated females or implantation of extra glands in virgin hosts was demonstrated, that not the adult Corpus allatum but the Median Neurosecretory Brain Cells (MNBC) are responsable for this germ cell multiplication.Adult host provided with larval Ring gland or pupal host accelerate the germ cell differentiation but not the germ cell multiplication of implanted testis. This acceleration would be much higher if the normal development of the germ cells in adult hosts were due to rests of pupal hormons in this milieu.The nutritive conditions of the host play a very important role in germ cell multiplication. The testis implanted in castrated females or females injected with casein hydrolisate show a higher number of developing germ cells. The MNBC controll the protein metabolism (assimilation) but are only indirectly responsable for the germ cell multiplication; the multiplication is a consequence of the autonomous protein synthesis of the germ cells themself.The growth of other imaginal discs in adult hosts are discussed in the light of these results.