202 REGENERA TION 



that gives rise to the digestive tract, nervous system, and middle-layer 

 cells in the new parts. From them also arise the new pharynx, and the 

 lining of the pharynx chambers, as well as some other structures. It 

 is impossible to say at present whether one and the same kind of cell 

 may give rise to all these structures, or whether different kinds of 

 cells are present in the middle layer, that cannot be distinguished 

 from each other by the methods at present at our command. 



The changes taking place in the tissues of those animals that 

 regenerate by morphallaxis have been only quite recently carefully 

 investigated. Bickford stated that in tubularia the old differentiated 

 tissue changes over directly into the tissue of the new part, and 

 Driesch confirmed this statement. Stevens has studied by means of 

 serial sections the different changes that take place. Division of both 

 ectodermal and endodermal cells is found to occur, but especially the 

 ectodermal. Whether all the ectodermal cells divide, or only some of 

 them, is difficult or impossible to state, but whether this happens or 

 not, all the old region goes over into the new hydranth. 



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. I, 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. 1 The experiment seemed to show that the two layers 

 could change their specific character and be transformed into 3ach 

 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 



1 How the tentacles could have gotten into their normal position is not explained. 



