The amphibianorganizercorresponding to the upper dorsalblastopore lipof the earlygastrula, is one of the classical systems in which embryonic patterning has been studied (1 ). Its two major inducing activities areneural induction and dorsalizationof ventralmesodermdifferentiation. Organizer activity is revealed either by transplantation of the upper dorsal blastopore lip into the ventral side of a host gastrula or by inserting the tissue into the gastrula blastocoel (Einsteck method). This results in the formation of an induced structure which, depending on the embryonic stage of the upper blastopore lip, is a head, trunk, or a tail. While the Einsteck is much easier to perform than the organizer transplant, the position of the implant can be less well controlled, which needs to be considered when interpreting the results (2 ). However, the Einsteck can be also carried out with heterologous material, cell pellets, and injected animal caps (3 ,4 ) and is more versatile than the organizer transplant.
