PLATE 2 



EXPLANATION OF FIGURES 



9 Reconstructed Transverse section of a vertebra of a normal control frog 

 killed March 28, body length 56 mm. showing the great amount of periosteal 

 bone and osteogenic tissue. 



10 Reconstructed Transverse section of the vertebra of a thyroidless tadpole 

 killed February 7, total length 123 mm., body length 43 mm showing the poorly 

 developed periosteal bone, the small amount of endochondral ossification and 

 the primitive condition of the cartilage cells of the dorsal bridge and neuro- 

 central suture. 



11 Transverse section through the neural arch of a normal control frog 

 killed March 28, body length 56 mm., showing the well developed periosteal 

 bone and the large mass of osteogenic tissue. 



12 Transverse section of a thyroidless tadpole killed March 6, total length 

 102 mm., body length 40 mm. showing the poorly developed periosteal bone and 

 the comparatively small amount of endochondral ossification. 



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