690 C. M. LAPIERE AND J. GROSS 



than in the controls, whereas in the tail fin the reverse occurred. 



In the back skin there was an increase in both total and specific 

 activities in the metamorphosing animals as compared with the 

 controls, suggesting increased synthesis. The elevated specific ac- 

 tivit}' in the face of diminished total collagen suggests a preferential 

 attack on the older "cold" collagen. 



Partition of the dift'erent collagen fractions differed markedly be- 

 tween tail fin and back skin, and the influence of thvroxin on this 

 pattern in the two tissues was also dissimilar. 



In the resorbing tail a striking alteration was noted in the relation- 

 ships of the net incorporation in the different fractions. The specific 

 radioactivitv of tlie neutral-extractable fraction was well below that 

 of the insoluble collagen and was also considerably lower than in 

 the corresponding control. A rise in specific activity, coupled with 

 a diminished amount of insoluble collagen, appeared to result from 

 preferential removal of an unlabeled, hence older, portion of a meta- 

 bolically inhomogeneous pool. 



We speculate from these metabolic studies that the old insoluble 

 collagen framework is preferentiallv removed and replaced continu- 

 ously by a new insoluble scaffolding. The newly deposited fibrils 

 may be protected in some manner from attack bv the enzyme. Per- 

 haps the ground substance, produced in concert with new collagen, 

 mav provide a protective coat. Alternatively, the older fibers are 

 attacked first because of geographic proximity to the cells with lytic 

 power. 



Collagenase activity was tested by culturing small bits of tissue 

 on a reconstituted fibrous collagen substrate. Living tissues were 

 required for collagenolytic activity. Of twelve different tadpole or- 

 gans examined for collagenolytic activitv, four were active, namely 

 tail fin and back skin, gill, and gut, i.e., organs which undergo the 

 most dramatic remodeling and resoi^ption of connective tissue during 

 metamorphosis. 



Where rapid collagen removal occurred, as in the resorbing tad- 

 pole tail, there was an increase in degradative activitv. This effect 

 might simply be due to a concentration of enzyme-producing cells 

 as a result of tissue water loss. The increased lysis of collagen may 

 be ascribed to the invasion of the collagenous basement lamella by 



