EFFECTS OF EXTIRPATION OF THYROID GLAND 579 



animal we see several marked differences. In the thyroidless 

 animal we have again only the three centers of ossification pres- 

 ent, but in the control in addition to these three centers we also 

 find ossification taking place in the calcaneum and astragalus 

 bones. The bands of ossified bone are considerably larger, 

 and the digits are almost perfectly formed at this early stage. 

 So again it seems just to conclude that the removal of the thy- 

 roid gland has greatly retarded if not completely stopped the 

 process of ossification as well as the process of growth. 



SUMMARY 

 1. Control tadpoles 



The process of calcification is almost complete by the begin- 

 ning of metamorphosis. By the time metamorphosis is completed 

 the process of ossification has proceeded so far that the calcified 

 cartilage has become markedly reduced by absorption. The 

 process of ossification continues until by March 28th the neuro- 

 central suture has become almost obliterated and both per- 

 iosteal and endochondral bone are well developed. There is, 

 however, at that time a total absence of ossification in the ribs. 

 The spinous process is well developed by the time of metamor- 

 phosis, but calcification does not appear in it until later. 



2. Thyroidless tadpoles 



The vertebrae of the thyroidless tadpoles continue to in- 

 crease in size long after the period when the controls have meta- 

 morphosed. Although the vertebrae are strictly larval in 

 character, both in form and as regards their very early stage of 

 ossification, nevertheless they continue to grow in size in cor- 

 respondence with the growth of the body, becoming much 

 larger than the vertebrae of the controls that had undergone 

 metamorphosis; but retaining characters far more primitive 

 than those shown by the latter. This is especially seen in the 

 absence of the spinous process even in the tadpole killed March 

 6. The cartilage becomes extensively and rather uniformly 

 calcified, invading a large portion of the neural arch. The 



