The presumptive material of pronephros was exchanged betweenBombina variegata andTriturus alpestris in both gastrula and neurula stages. In further series the pronephros material was taken from the neurula stage of the two orders, dissociated mechanically and cultured in an ectodermal envelope (sandwich technique). This showed that the differentiation proceeds autonomously. No structures other than that corresponding to the prospective significance of the transplanted tissue have been found. Order-specific characters such as the rate of differentiation and the nuclear arrangement in the nephridial epithelial cells were maintained by both tissues. Chimeric nephridial tissues could not be observed. 2. The pronephros tissue of a foreign order, when used as a donor, enters into the host's coelom and can be replaced reciprocally. This leads to the formation of a nephrostome. In this case the developmental-physiological factor is apparently common in both amphibian orders. 3. The analysis and reconstruction of the pronephros inBombina andTriturus embryos of different ages showed that the formation of three nephrostomes in anurans and two in urodeles is only temporarily valid forBombina. Eight days after neurulation (PM24) only the two caudal nephrostomes persist inBombina. 4. The interference to the pronephros function due to operation leads to hypertrophy of the intact kidney on the control side in older embryos.
