572 GEORGE S. TERRY 



that the osteoblasts are first formed in the center of the neural 

 arch and it is here that the first dissolving away of the salts takes 

 place. As the process continues they gradually work their 

 way toward the outside of the vertebrae until they reach the 

 periosteal bone where the process appears to stop. 



In making a comparison of the thyroidless and control ani- 

 mals let us first consider figures 1 and 2. These represent 

 end views of the third vertebrae. Figure 1 represents the 

 vertebra of a normal control frog killed August 6, body length 

 21.5 mm. and figure 2 represents the vertebra of an oper- 

 ated tadpole killed October 14, total length 80 mm., body 

 length 29 mm. In comparing these two individuals one first 

 notices the difference in size of the two vertebrae and also 

 a great difference between them in the extent of the calcifica- 

 tion of the cartilage in the centrum and neural arch regions. 



In comparing these two it is seen that the vertebra of the 

 thyroidless animal is considerably larger than that of the con- 

 trol, but this increase in size is no doubt due to the fact that the 

 thyroidless animals have continued to grow in size after ceasing 

 to differentiate and are actually larger than the normal tadpoles 

 at the time of metamorphosis. In comparing the centra of 

 the two individuals we find the most striking difference. The 

 centrum of the thyroidless tadpole killed October 14, is more 

 than a third larger than is the centrum of a control killed Aug- 

 ust 6, while actual measurements show that the percentage of 

 deposition of calcium salts of the control compared with that 

 of the thyroidless specimen is as 5 to 9. This numerical com- 

 parison is made by taking an average of the thickness of the 

 calcified portions of the centrum. 



In the metamorphosed control there has been a marked de- 

 gree of absorption of the calcified cartilage. 



In comparing the thyroidless tadpole which was killed March 6, 

 total length 102 mm., body length 40 mm. with the control frog 

 killed March 28, body length 56 mm. we find that there has been 

 continued growth of the vertebrae of the normal frog until they 

 have outstripped those of the thyroidless tadpoles. This is in 



