688 C. M. LAPIERE AND J. GROSS 



in concert with new collagen, may provide a protective coat. There is 

 some evidence to indicate that newly formed collagen (reticulin) is 

 surrounded by ground substance not found in older mature collagen 

 bundles (Robb-Smith, 1957). At this stage of the game there are 

 other equally plausible hypotheses. Hay and Revel (1963), using 

 radioautography, have noted that newly deposited collagen first be- 

 comes evident in the regenerating basement lamella adjacent to the 

 epithelial cells. During thyroid-induced tail resorption we (Usuku 

 and Gross, unpublished observations) noted the earlv invasion of 

 the deeper layers of the lamella by mesenchymal cells with local 

 collagen disruption; only later were the more peripheral, presumably 

 newly deposited, collagen layers attacked. There is also the possi- 

 bility that the spreading apart of the deeper collagen layers early in 

 resorption is a result of a shift of water into this layer. Loosening 

 of the structure might then facilitate the entrance of the mesen- 

 chvmal cells. 



The differences in collagen turnover between back skin and tail 

 fin suggest that the same stimulus, thyroxin, affects skin in two 

 adjacent regions in different ways. There was no detectable increase 

 in collagenolytic activitv in the back skin, nor was there nearly the 

 same degree of dehydration. The net incorporation of label in the 

 back skin of thyroxin-treated animals was greater than that in the 

 normal, in sharp contrast to the picture in the tail tissues. We know 

 from older studies such as those of Clausen (1930) that body skin 

 transplanted to the tail does not resorb during metamorphosis, 

 whereas tail fin transplanted to the back undergoes resorption at 

 the same rate as in its parent site. There may be some difference 

 in the properties or composition of the collagen of the tail fin and 

 the back skin to account for this observation. Certainly there is a 

 marked contrast in collagen concentration and distribution be- 

 tween these two sites. Thus far, examination of denaturation tem- 

 perature indicates that the stability of the acid-extracted collagen 

 in the two regions is identical, and even young and old bullfrog 

 collagens are no different in this respect. Perhaps other analyses 

 may demonstrate a difference which could alter susceptibility to 

 collagenase. However, it seems more likely that the cells responsible 

 for the reorganization of the tissues in different regions respond 



