EFFECTS OF EXTIRPATION OF THYROID GLAND 571 



The first indication of the effect of the removal of the gland 

 is shown by the fact that the tadpole fails to metamorphose and 

 maintains its larval characteristics after it has passed the period 

 when the change should have taken place. The head becomes 

 flattened dorsoventrally, the tail fails to shorten, and the hind 

 legs are greatly delayed in development. In tadpoles three 

 months past the period of metamorphosis the hind legs have 

 reached a length of only 4 to 5 millimeters all of which clearly 

 goes to show that the removal of the gland has produced a very 

 great effect upon the development of the body beginning either 

 at or probably before the period of metamorphosis. A com- 

 plete account of the external characteristics of growth has been 

 given in the previous article by Allen 



Before attempting to make a comparison of the deposition 

 of the calcium salts in the thyroidless and control specimens 

 let us consider the normal process of deposition. What is the 

 nature of the salts, what part do they play in bone formation 

 and what relation do they bear to the centers of ossification? 

 In the normal control animal the process of the deposition of 

 the calcium salts begins at three points in the vertebra, viz., 

 the centrum, neural arch and transverse process. These are 

 known as deposition centers and later become centers of ossifica- 

 tion. The first change in the cartilage takes place in these 

 centers, the cartilage cells increase in size and in number in 

 such a way that several enlarged cartilage cells come to lie in 

 a single enlarged cell space. The cells now arrange themselves 

 in rows, there is an increase in the intercellular matrix and a 

 deposition there of calcium salts. By this process the cartilage 

 becomes calcified and the areas involved are known as calci- 

 fication centers. 



Ossification proper begins by blood vessels from the peri- 

 osteum pushing their way into the calcified cartilage at the cal- 

 cification centers carrying with them some of the osteogenic 

 tissue from beneath the periosteum. The blood vessels carry- 

 ing this tissue arc known as periosteal buds. Osteoblasts now 

 develop from the osteogenic tissue and dissolve the calcified 

 cartilage. Free hand sections cut through the vertebrae show 



