REGENERATION IN VERTEBR.\TES 291 



and 6). The cortical cytoplasm contains one or more polyps which 

 extend into the ground substance that bathes the undersurface of the 

 epithelium. Their size varies; the largest may exceed a micron, and the 

 smallest, often very numerous, constitute only surface irregularities. In 

 some instances the extrusions are shaped like fingers, but more fre- 

 quently they are rounded with a broad base. Sometimes the entire ex- 

 posed surface of the cell is involved; in still others, only a part. The very 

 tip of the extensions are often torn or broken, and the marginal cyto- 

 plasm appears to merge with the underlying ground substance. The sur- 

 face break is often multiple, and so the stream is broken here and there 

 by a persistent remnant of the plasma membrane. However, the ap- 

 parent breaks may not be real but only distortions introduced by fixa- 

 tion and subsequent treatment. 



The cytoplasmic protrusions are most abundant in distal-most 

 parts of the early regenerative stages at the time of formation of the 

 blastema, but they are occasionally seen in more advanced stages. 

 They are seen after potassium permanganate or osmium tetroxide 

 fixation, and as we have already noted, the larger ones are observed 

 with the light microscope after fixation in Bouin's fluid. Their presence 

 may be a morphological sign of the participation of epidermal cells in 

 the events of regeneration of the organ. Perhaps the protrusions con- 

 stitute a secretory contribution to the blastema. We have already sug- 

 gested that the relatively voluminous endoplasmic reticulum often 

 seen in epidermal cells may reflect increased secretory activity, and 

 that the enlarged spongy cortical zone may represent the accumulated 

 secretion. Perhaps the protrusions are localized eruptions by which 

 substances leave the cell to penetrate underlying regions, and in this 

 way the epidermis may influence regeneration of the body part by 

 means of chemical agents. The possible role of such agents will be 

 speculated upon below. 



Concerning the question whether direct physical contact between 

 the bare epidermis and the underlying mesenchymatous tissue is a pre- 

 requisite of regeneration, it is important to note at this point that often 

 (approximately 30 per cent of the time) there forms at the boundary 

 between epithelium and mesenchymatous tissue a fluid blister which 

 separates the two, distorts the shape of the regenerate, and persists 

 during the stages of rapid growth (see also Rose, 1948a). The regen- 

 erate and adjacent stump are edematous during the formative stages, 

 and the edema fluid is ordinarily dispersed, causing a general swell- 

 ing in stump and regenerate tissues. Formation of a blister, which 

 seems to occur suddenly, apparently results from localized concentra- 

 tion of the fluid. The boundary is a favored locus of blister formation, 

 and as the vesicle forms, it dissects the margin between epidermis and 

 blastema. Sometimes handling of the animal and mechanical dis- 



