EFFECTS OF EXTIRPATION OF THYROID GLAND 575 



develops very early in the formation of the vertebrae. Figure 

 5 shows that its cartilaginous anlage is well developed even at 

 the time of metamorphosis and probably before. In the con- 

 trol frog killed March 28 (fig. 9), not only the lateral portion 

 of the neural arch, but the dorsal bridge as well is completely 

 ossified and the primitive cartilage cells have been replaced by 

 endochondral and periosteal bone. 



In the thyroidless tadpole killed Oct. 14, total length 80 mm., 

 body length 29 mm. not even the rudiments of a spinous process 

 are developed, and even in specimens which are seven months 

 past the period of metamorphosis it is not yet evident. Com- 

 parison of the cartilaginous bridges shows that the control is 

 in a more advanced stage. The cartilage of the operated tad- 

 pole is of a very primitive hyaline type and appears merely as 

 a gelatinous mass. In specimens killed four months past the 

 period of metamorphosis it is in a somewhat more advanced 

 stage but the amount of calcification is almost negligible. 

 Measurements of both operated and control specimens which 

 were killed two to three months after metamorphosis fail to 

 show any marked differences in the width of the dorsal bridge. 



MICROSCOPICAL STUDY 



Now that it has been shown through the study of whole 

 mounts that there is a marked difference between the deposi- 

 tion of calcium salts in the thyroidless and control specimens, 

 it becomes of interest to know their histological nature. 



After a careful study of the vertebral column of the thyroid- 

 less and control animals it was found that the third vertebra 

 might be taken as a typical example and it is for this reason 

 that reference will be made to this one in particular. 



Histological study of the vertebral column of a normal con- 

 trol tadpole shows that the process of ossification begins very 

 early. Figure 13 represents the amount of ossification present 

 when the hind legs have reached a length of only twelve mil- 

 limeters. At this early stage of development very little en- 

 dochondral ossification of the centrum was noticed but the ossifi- 

 cation of the neural arch was well under way. Figure 9 



