678 



C. M. LAPIERE AND J. GROSS 



CONTROL 



THYROXIN Rx 



J I I L_ 



20 30 10 20 



HOURS AFTER PROLINE - H^ 



30 



Fig. 10. Specific activity of the three collagen fractions isolated from back skin 



ble fraction is by far the largest, the acid-extractable next, and the 

 neutral-extractable the least. The situation in the back skin of the 

 tadpole is very similar, but that in the tail fin very different, as noted. 

 There is a striking difference in radioactivity distribution. In mam- 

 mal skin the acid-extractable fraction is labeled to a greater degree 

 and more rapidly than the insoluble, although much less than the 

 cold neutral-extractable moiety. In contrast, in the tadpole the 

 acid-extractable collagen of both tail and back skin is tlie least and 

 slowest labeled. The shape of the curve closely parallels that of the 

 insoluble fraction rather than that of the neutral-extractable com- 

 ponent. 



Another striking feature of the data is the nearly equal rate of 



