REGENERATION IN VERTEBRATES 293 



intruded deeply and encompassed the deviated structures as well as a 

 blood clot that formed at the site of operation. He noticed islets of 

 epidermal cells which he believed had lost contact with the remaining 

 epithelium by internal migration (see also Ide-Rozas, 1936; Roguski, 

 1953). Taban affirmed Rose's view of internal movement and dissocia- 

 tion of epidermal cells but denied that the cells formed elements of 

 the blastema. More recently Bodemer (1958) saw streamers of epi- 

 dermis consisting of 30 or 40 cells extending deeply into the underlying 

 musculature in the early stage of supernumerary limb formation in the 

 newt. Roguski (1953) reported that cells became detached from the 

 epithelium as it moved over the amputation wound of Xenopus tad- 

 poles and lay in crevices; he believed that the islands do not generate 

 other tissues but, instead, die and disintegrate. "Introverted projec- 

 tions" of the epithelium extending deeply into wound tissues were 

 encountered by Schmidt ( 1958a) in thyroidectomized animals. 



Recent results from our laboratory on beryllium-poisoned regen- 

 erates (Scheuing and Singer, 1957) are instructive in the analysis of 

 the relation of epidermis to wound tissues. There was widespread de- 

 struction of regenerate and stump tissues but the epidermis itself re- 

 sisted the poison; indeed, it appeared to be stimulated to high activity 

 and even to participate in the destructive processes. Large projections 

 of epidermis extended deeply within the dying tissues, and it was diffi- 

 cult to determine the boundary between epidermis and other tissues at 

 the ends of these extensions, because the cells of the epidermis ap- 

 peared to be dissociated and separated by the considerable debris 

 (discussed further below). One might interpret the intermingling of 

 cells and debris to mean that epidermal cells leave the basal layer of 

 the intrusions to move more deeply individually or in clusters. The 

 beryllium studies showed that the thickness, the irregvilarities in shape, 

 and the over-all disposition of the epidermis are related directly to 

 underlying events. 



A striking evidence of the sensitivity of the epidermis to the 

 mechanical and chemical composition of wound tissues and blastema 

 occurs when various tissues or inert substances are implanted into the 

 early regenerate of the newt. The epidermis immediately grows around 

 the foreign object ("seizes" it, so to speak), as though to exclude il, 

 and then sometimes throws it out. For example, implanted spinal gan- 

 glia taken from the same animal were almost completely enclosed 

 within a day or two by deep ingrowths of the epidermis ( Kamrin and 

 Singer, 1959). But then the response subsided and the projections 

 were retracted. We have obtained the same reaction \vith pieces of 

 normal and predegenerated nerve and of normal skin (Figure 4). A 

 greater reaction was evoked by dead newt ganglia and ganglia taken 

 from the frog. The epidermis completely encompassed the implant to 



