The task of the Stammaim was to complete what was missing (usually through conjectural restoration) and to integrate the whole into a flowing discourse. They reserved for themselves the right to preface, conclude, and even interpolate the words of the Amoraim; otherwise, they could not have integrated and reconstructed them. The state of some of the argumentational material that survived was such that it required the intervention of the Stammaim at almost every turn. As a result, it is often very difficult to distinguish between what belonged to the Amora and what was added by the Stammaim, since the two are often interwoven. I did not exaggerate, therefore, when I said elsewhere that between us and the Amoraim stand the Stammaim.
David Weiss Halivni, Midrash, Mishnah, and Gemara: The Jewish Predilection for Justified Law (Camridge, MA & London, UK: Harvard University Press, 1986), 77.