REGENERATION IN VERTEBRATES 295 



ber of weeks are required before implanted paraffin pieces break 

 through the skin. Glass beads and paraffin cones were retained for 

 long periods in the Axolotl ( Orechowitsch and Bromley, 1934 ) . When 

 the amputation stump was pushed into a skin pocket of the body wall 

 ( Polezaiev and Faworina, 1935 ) , it did not break through the envelop- 

 ing skin but was retained indefinitely. 



Overton (1950) implanted methylcholanthrene crystals, paraffin, 

 and glass beads under the intact skin of the dorsal fin of larval Ambly- 

 stoma. The implants were extruded in most cases within a few days. 

 Histological studies showed early breakdown of the basal membrane, 

 inward migration of cords of epidermal cells, walling off, and extrusion 

 of the implant. During this time there was no increase in mitotic count. 

 The fact that normal epidermis can be excited to throw out a foreign 

 body shows that this reactivity is a common property of epithelia, al- 

 though more highly developed in that of the regenerate. 



Gross epithelial movements in response to the presence of a for- 

 eign body, such as we have seen, have also been reported for inverte- 

 brates (Lazarenko, 1928; Danini, 1928; Zawarzin, 1927). Zawarzin im- 

 planted a small celloidin tube between the epithelial layers of the 

 mantel in the mollusc Anodonta; the tube became surrounded by a 

 continuous sheet of epithelium and then was carried to the surface. 

 The epithelial reaction was preceded and partly accompanied by a 

 connective tissue reaction. At first two enveloping layers of connective 

 tissue were deposited, the second of which formed a syncytium. The 

 connective tissue cells degenerated, leaving a ground substance. The 

 capsule evoked an inward growth and a secondary encapsulation by 

 the epithelium. Wounding of the basal membrane was prerequisite for 

 the epithelial response, because it was only from the locus of such 

 damage that the epithelium moved inward. Similar results were ob- 

 tained in the same laboratory by Lazarenko ( 1928 ) on the beetle and 

 by Danini ( 1928 ) on the crayfish, and the encapsulated celloidin was 

 expelled in a subsequent molt. In our experiments on the newt the 

 epithelium responded more quickly and did not require a prior con- 

 nective-tissue response. 



In addition to encapsulation of large bodies, the epidermis draws 

 into its intercellular spaces a continuous stream of wandering cells 

 and debris of various sorts, without gross alterations in its contours. In 

 normal limb regeneration we observed debris of various sorts and 

 wandering cells among the epidermal cells. These do not exist in great 

 abundance but are found frequently enough to be seen on occasion 

 even in the restricted field of the electron microscope. The degenerat- 

 ing cells and cellular debris may also be dead epidermal cells and cellu- 

 lar fragments in addition to subepidermal elements. 



Other workers have also observed the movement of cells and vari- 



