292 CELLS, TISSUES, AND ORGANISMS 



turbance of the stump seem to be enough to provoke vesiculation. The 

 bHsters in about half the instances are very large— many times the vol- 

 ume of the regenerate itself— and stretch the overlying epithelium into 

 a thin, transparent layer of a few cells. The contents of the blister are 

 clear except for occasional bleeding into it. In sections a small amount 

 of debris and occasional cells may be seen within the blister. Although 

 the vesicle efiFectively separates the epithelium and mesenchymatous 

 parts, regeneration does not cease, nor does it seem to be slowed. In- 

 deed, the vesicle is invaded from below by mesenchymatous cells 

 which gradually replace the fluid. Consequently, immediate contact 

 between the two tissues, at least during the period of early bud forma- 

 tion, is not required for later regenerative events. Blister formation 

 does not, of course, preclude chemical contact between epithelial and 

 mesenchymatous parts by diffusion through the blister fluid. 



In summary, our morphological studies, particularly those with 

 the electron microscope, reveal no unbroken barrier to chemical and 

 physical communication between epidermis and blastema. They also 

 show that the epidermal cells of regenerates differ mainly in a quanti- 

 tative way from that of normal. The endoplasmic reticulum is more 

 elaborate and shows more signs of protein secretion; there are more 

 mitochondria; there is an abundant outer cytoplasmic zone devoid of 

 morphological structure; and the cells are more loosely arranged. 

 Finally, there are morphological signs of chemical contribution of the 

 basal cytoplasmic layer of the epidermis to underlying tissues. 



Massive epithelial movements and phagocytosis 



A number of workers have remarked upon the fact that irregulari- 

 ties may exist in the deep contour of the epidermis in contact with 

 wound and regenerating tissues. Rose (1948a) considered the irregu- 

 larities a sign of cellular dissociation and movement of epidermal cells 

 inward to form the blastema. In our experience, irregularities arise 

 especially when the contour of the amputation surface is particularly 

 uneven, since the epidermis in its growth applies itself closely to the 

 wound tissues. Wendelstadt ( 1904; see also Ide-Rozas, 1936 ) observed 

 that when bone of the amputation stump protruded from the surface, 

 the epidermis was sometimes unable to cover it but piled up at its 

 base and formed a thick mound of epithelium. Naville ( 1924) depicted 

 variations in the contour and thickness of the regenerate epidermis. 

 Chalkley (1954, 1959) remarked upon these irregularities and dis- 

 agreed with Rose that they are areas of cellular dissociation. Taban 

 ( 1955 ) observed striking irregularities in the basal layer of epidermis 

 when he deviated the brachial artery and associated sympathetic 

 nerves of the salamander to the body wall. The regenerating epidermis 



