The testes ofEphestia kühniella go through two morphogenetic processes during metamorphosis: fusion of both organs in the prepupa (stage A3) and torsion in the early pupal phase (day 1-5; 23° C). 2. By ligating behind the thorax the critical periods of these processes have been determined: for fusion the transition to prepupal phase, for torsion the second half (A3-A4) of this developmental stage. 3. The humoral conditions during prepupal phase do not enable testis to make an autonomous torsion. This process is connected to a pupal milieu. 4. When testes from donors, which have not yet passed the critical periods of fusion and torsion, are directly transplanted into pupae differentiation will only occur, if they are exposed to the influence of secretions of endocrine organs during this stage. The effectiveness of the morphogenetic factors decreases with aging of this stage and is no more detectable towards its end. 5. Testes of various ages show different responsiveness to an effective humoral milieu: the younger the grafts the later they respond. Investigations with progressively younger larvae, however, did not yield a threshold of competence until the 3. instar (younger larvae withdrew from operation because of their small size). 6. On the base of these results a conception of the control of testis differentiation during metamorphosis is traced. It is suggested that beside the stimulus of prothoracic gland hormone during the prepupal stage a second, pupal factor is necessary for torsion.
