576 GEORGE S. TERRY 



represents a reconstructed drawing of the third vertebra of 

 a normal control frog killed March 28. In this individual we 

 see that we are dealing almost entirely with periosteal bone and 

 osteoblastic tissue. The numerous cartilage cells present in 

 the earlier stages (fig. 13) have been almost completely broken 

 down and the few cartilage cells that remain are only to be 

 found at the ends of the rib and in the region of the centrum. 

 There no longer exists a definite neuro-central suture and dor- 

 sal bridge but they have both become a part of the neural arch 

 and the primitive cartilage cells present at the time of meta- 

 morphosis have been replaced by bone. 



In the thyroidless tadpole there is a great retardation in the 

 process of ossification and the operated animal killed October 

 14 compares very closely with the control killed before meta- 

 morphosis (fig. 12 and 13). In the operated tadpole killed 

 February 7 (fig. 10 reconstructed) the periosteal bone is very 

 poorly developed and we are dealing almost entirely with a 

 mass of hyalin cartilage cells which in the centrum have been 

 invaded only in a few places by endochondral bone spicules. 

 The line separating the centrum and the neuro-central suture 

 is very plain and the cartilage cells of the latter are of a primi- 

 tive hyalin type. There also appears to be a marked retarda- 

 tion in the development of the periosteal bone, it appears as a 

 very thin layer surrounding the outside of the arch and diffuses 

 into the hyalin cartilage cells in the region of the dorsal bridge. 



In our study of the whole mount of the operated tadpole 

 killed February 7 we especially emphasized the fact that the 

 width of both the neuro-central suture and the dorsal bridge 

 had been greatly reduced. Microscopic examination shows 

 that both the neuro-central suture and the dorsal bridge are 

 much wider than was previously interpreted. The cartilage 

 cells appear to have reached a comparatively late stage in cal- 

 cification but as yet no sign of ossification is present. 



Figures 15 and 16 represent cross sections through the neural 

 arch and serve merely to show further the great retardation 

 in the process of ossification in the operated specimens. 



Let us now compare the operated tadpole killed October 



