Abraham Trembley of Geneva (1710—1784), while employed as a tutor-in-residence for the two children of Count Bentinck of the Hague in Holland, rocked the scientific world with his discoveries concerning the “? Polyp with arms shaped like horns,― later designated as the genus Hydra by Linnaeus in 1746. In addition to carrying out the first grafts ever made using animal tissue, Trembley, while using hydra, made a number of other majorbiological discoveries (see Baker, 1952). During four productive years, Trembley proceeded to: (a) perform the first rigorous experiments on regeneration in animals; (b) prove that asexual reproduction can take place in animals by budding; (c) demonstrate that tissues can be vitally stained and that these stained tissues can be used in experiments; (d) describe properties of materials which fit Dujardin's description of protoplasm reported almost 100 years later in 1835; and (e) show that “? eyeless― animals can respond to light. Although the technique of grafting animal tissues together is widely used in ex perimental biology and in medicine, few scientists know how and where the technique originated. Interestingly, among those who rightfully credit Abraham Trembley of Geneva with carrying out the first animal grafts on the freshwater hydra (Trembley, 1744), some report incorrectly how the very first of these grafts were made (e.g., Baker, 1953). In this review we describethe firstanimal graftsas presentedby Trembley in his MA©moires (1744), and show how they originated from his experiments on turning hydra inside out. Where appropriate, we use Trembley's own words in direct translation or in paraphrase,together with copies of the original illustrations published in 1744. By describing the details of his experiments and the thoughts behind them, we hope to introduce many biologists, who are but dimly aware of Trembley, to a remarkable figure in the history of biology. Unlike most of his peers, Trembley conducted and reported his experiments with a detail, caution, logic, and rigor rare for his time. In recognition of his astounding discoveries, he was elected to the Royal Society of London and in 1743 was awarded its prestigious Copley Medal, considered then to be one of science's highest honors. Finally, although virtually all the observations and experiments reported in Trem bley's MA©moires (1744) have been confirmed over the past two centuries and still serve as models for much research in experimental morphology and developmental biology, the processes taking place at the cell and tissue levels that underly many of his observations remain to be elucidated. Some of these unanswered questions become apparent as we discuss the classic grafting experiments Trembley performed in the fall of 1742.