578 GEORGE S. TERRY 



of the control is of a late fibrous or early hyalin type. Our 

 study would therefore seem to indicate that the removal of the 

 thyroid gland has produced a general retardation in the process 

 of ossification which is not confined to any one part of the verte- 

 bral column in particular. 



STUDY OF THE HIND LEGS 



A study of both thyroidless and control individuals shows 

 that the same principles which were applicable to the bone 

 formation of the vertebral column are likewise applicable to the 

 bone formation of the hind legs, and especially is it to be em- 

 phasized that the retardation of ossification in the bones of the 

 hind legs is much more marked than in the case of the vertebrae. 

 Even so great is the retardation that one is almost justified in 

 saying that the process of ossification after it has reached a cer- 

 tain stage is completely stopped. 



The results can best be studied by referring to figures 17 and 

 18. Figure 18 represents the 12 mm. hind legs of a normal 

 control tadpole killed before metamorphosis while figure 17 

 represents a 4.5 mm. hind leg of an operated tadpole killed 

 October 14. 



In the control tadpole having a leg length of 12 mm. we find 

 that the bones of the hind legs are completely formed although 

 still in a very early cartilage state. The tibia-fibula is joined 

 only at the epiphysis. The centers of ossification are clearly 

 marked and the osteogenic tissue has invaded the cartilage at 

 the extremities of the bones and has given rise to the well marked 

 epiphyses. 



In the operated tadpole we have only three marked centers of os- 

 sification present, and these are found in the femur, tibia and fibula. 

 The tibia and fibula are completely separated as is also the as- 

 tragulus and the calcaneum due to the absence of the epiphysis 

 at this early stage. Only four of the digits are present and these 

 are represented by only one bone in each with the exception of 

 the fourth which has two. 



In comparing the 5 mm. hind leg of the thyroidless tadpole 

 killed March 6 with the 6 mm. hind leg of a normal control 



