Mainly devoted to the investigation of the elementary structure of animal tissues, Schwann’s Untersuchungen had an important bearing on the development of the doctrine of the cell structure of animal tissue. In this work Schwann discarded Schleiden’s Uhrglastheorie and put forward a theory of his own. In the same work he described the neurilemma, the “sheath of Schwann”. Schwann was Professor of Anatomy and Physiology at Liège. English translation (Sydenham Society), 1847. See M. Florkin, Naissance et déviation de la théorie cellulaire dans l’ceuvre de Theodore Schwann. Paris, Hermann, 1960. Digital facsimile of the 1839 edition from Google Books at this link.