The changes that take place in hydra have been recently worked out in my laboratory by Rowley, who finds that a certain amount of division takes place in the old cells, especially in the ectoderm. The division of the cells is not a very active process, and it seems not improbable that many of the old cells go over without dividing into the new part. One of Trembley’s most celebrated experiments was that in which hydras were turned inside out (Fig. 1, _A_, _B_), so that the ectoderm came to line the inner cavity and the endoderm to cover the outer wall. The tentacles were not everted but remained sticking out of the mouth of the everted animal. Their openings, or arm-holes, therefore, appear on the outer surface of the body. In order to prevent the everted hydra from turning itself back again, as it tends to do, Trembley pushed a small bristle crosswise through the wall of the body. Finding the hydras still sticking on the bristles the next day, he concluded that they had not returned to their former condition, but that the outer layer (the endoderm) had changed its character so that it became ectoderm, and the inner layer (the ectoderm) became endoderm.[98] The experiment seemed to show that the two layers could change their specific character and be transformed into each other according to their position in the animal. These remarkable results were not challenged until 1887, when Nussbaum repeated the experiment and showed that Trembley had overlooked an important fact. It was found that even the bristle pushed through the body does not prevent the hydra from regaining its original condition, although it may delay the turning back. If the turning back can be prevented, the animal dies. Nussbaum showed how the turning-back takes place in an animal while it remains on the bristle. The everted foot-end begins first to turn back, pushing into the central cavity. When it comes to the bristle it passes to one side of it, and continuing to turn back the foot passes out of the mouth, drawing the rest of the body after it.[99] The last act of the turning can take place only by tearing away through one or both sides, and this is often done. The bristle may still remain sticking to the body through one side, or even remain through both sides if the body has, after tearing through, healed up around the bristle. The process of turning back may take place quite quickly, and had been overlooked by Trembley, who trusted too confidently to the presence of the bristle sticking through the animal.