580 GEORGE S. TERRY 



centers of calcification remain distinguishable for some time. 

 The neuro-central suture becomes almost entirely filled in by 

 calcification. There is a heavy deposition of calcium salts in 

 the end of the rib. The cartilage of the dorsal bridge and of 

 the neuro-central suture is of a primitive hyaline type. 



The most striking feature of the vertebrae of the thyroidless 

 tadpoles is the almost complete absence of ossification. While 

 the normal controls clearly show both periosteal and endochon- 

 dral ossification at the time of metamorphosis, the thyroidless 

 tadpoles show only the faintest indications of these processes, 

 even 7 months after the metamorphosis of the controls. This 

 in spite of the fact that they have continued to grow far be- 

 yond the size attained by the latter at metamorphosis. 



3. The limbs in control and thyroidless tadpoles 



The removal of the thyroid gland has greatly retarded, if 

 not completely stopped both the process of ossification and the 

 process of growth in the bones of the hind legs. 



It is thus seen that while calcification of the cartilage has 

 proceeded in the absence of the thyroid gland, there has been 

 an extreme retardation in the process of ossification. 



