EFFECTS OF EXTIRPATION OF THYROID GLAND 



577 



14 with the one killed March (> (figs. 12 and 14). During the 

 period of five months which has elapsed there appears to have 

 been very little change in the process of ossification. The 

 centrum and neural arch of the operated tadpole killed March 

 6 compared with the one killed October 14 shows no marked 

 advance in ossification and as previously stated compares very 

 closely with the control killed before metamorphosis. It is 

 quite true that there has been somewhat of an advance in the 

 process of calcification, for example, the width of the neuro- 

 central suture and the dorsal bridge have been considerably 

 decreased in this later stage and the cartilage cells in this region 



A B 



Text fig. A Transverse section through the intervertebral disc of a normal con- 

 trol frog killed March 28 showing the character of the hyalin cartilage cells. 



Text fig. B Transverse section through the intervertebral disc of a thyroidless 

 tadpole killed March 6, total length 102 mm., body length 40 mm. showing 

 the character of the fibrous cartilage cells. 



show a somewhat more advanced stage in their development 

 but this advance is very slow indeed. In comparing the control 

 killed before metamorphosis and having a leg length of 12 mm. 

 with the normal frog killed March 28 one observes striking 

 changes. The dorsal bridge and the neuro-central suture have 

 become a part of the neural arch, the primitive cartilage cells 

 have been transformed into bone, ossification has continued in 

 its normal course and has almost reached a stage of completion. 

 So it seems fair to conclude that the removal of the thyroid has 

 produced a great retardation in the process of ossification, that 

 amounts almost to a cessation of the process. Another interest- 

 ing point is shown by a study of the cartilage cells of the inter- 

 vertebral disc (text figs. A and B). In case of the operated tad- 

 pole the cartilage is of a very primitive fibrous type while that 



