674 C. M. LAPIERE AND J. GROSS 



of collagen and percentage of the total obtained in sequential 

 neutral and acid extracts and in the insoluble residues of control 

 and thyroxin-treated tail fin and back skin. The amounts of collagen 

 in the different fractions of the tail fin were roughly similar, the 

 insoluble representing the smallest amount. In the back skin, on 

 the other hand, there was very little neutral-extractable collagen 

 and the insoluble fraction represented the largest portion. This latter 

 distribution is more characteristic of that found in terrestrial mam- 

 mals. 



Thyroxin treatment for 6 days ( in this experiment ) resulted in 50 

 per cent diminution in the amount of neutral-extractable and in- 

 soluble collagen per tail fin and a loss of 57 per cent in the acid- 

 extracted. By far the largest fractional loss was in the insoluble 

 fraction. In contrast, in the back skin there was an increase in neu- 

 tral- and acid-extractable collagen accompanied by a diminution in 

 the insoluble fraction. The net result was 53 per cent loss of total 

 collagen in the tail fin and 19 per cent loss in the back skin. Thus, 

 partition of the different collagen fractions differed markedly be- 

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

 pattern in the two tissues was also dissimilar. 



In all the isotope data we were struck by the fact that incorpora- 

 tion of labeled proline into the neutral-extractable and insoluble 

 collagen fractions occurred at nearly the same rapid rates. 



Examination of the radioactivity of the individual collagen frac- 

 tions in the tail fin discloses some interesting changes during meta- 

 morphosis. Net incorporation in the saline-extracted fraction of the 

 control animals was about three times as great as in the insoluble 

 residue (Fig, 7). In the resorbing tail a striking alteration is noted 

 in the relationships of the net incorporation in the different fractions. 

 The level of radioactivity of the neutral fraction was about equal to 

 that of the insoluble collagen and considerably lower than in the 

 corresponding control. The insoluble collagen net incorporation was 

 about the same in both groups. The acid-extractable collagen was 

 least labeled and its curye of incorporation resembled that of the 

 insoluble fraction. Thus, the 50 per cent lower net incorporation 

 seen in tlie total collagen of the metamorphosing tadpole tail fin 



